Exhibit 10.4

Published CUSIP Number:                                

CREDIT AGREEMENT

($200,000,000)

Dated as of December 21, 2005

among

GILEAD SCIENCES, INC.,

as the Borrower,

BANK OF AMERICA, N.A.,

as Administrative Agent, Swing Line Lender

and

L/C Issuer,

The Other Lenders Party Hereto

and

ABN AMRO BANK N.V.,

CITIBANK, N.A.,

DEUTSCHE BANK TRUST COMPANY AMERICAS,

and

MIZUHO CORPORATE BANK (USA),

as Co-Syndication Agents

BANC OF AMERICA SECURITIES LLC,

as

Sole Lead Arranger and Sole Book Manager

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CREDIT AGREEMENT

This CREDIT AGREEMENT is entered into as of December 21, 2005, among GILEAD SCIENCES, INC. (the “Parent” or the “Borrower”), a Delaware corporation, each lender from time to time party hereto (collectively, the “Lenders” and individually, a “Lender”), and BANK OF AMERICA, N.A., as Administrative Agent, Swing Line Lender and L/C Issuer.

WITNESSETH:

WHEREAS, the Borrower has requested that the Lenders provide a revolving credit facility, and the Lenders are willing to do so on the terms and conditions set forth herein;

WHEREAS, under the Loan Documents (as defined below), Gilead Sciences, Inc. will be the borrower, and certain subsidiaries of Gilead Sciences, Inc. will be guarantors, in each case, as set forth in the applicable Loan Documents;

NOW THEREFORE, in consideration of the mutual covenants and agreements herein contained, the parties hereto covenant and agree as follows:

ARTICLE I

DEFINITIONS AND ACCOUNTING TERMS

1.01 Defined Terms. As used in this Agreement, the following terms shall have the meanings set forth below:

“Administrative Agent” means Bank of America in its capacity as administrative agent under any of the Loan Documents, or any successor administrative agent.

“Administrative Agent’s Office” means the Administrative Agent’s address and, as appropriate, account as set forth on Schedule 10.02, or such other address or account as the Administrative Agent may from time to time notify to the Borrower and the Lenders.

“Administrative Questionnaire” means an Administrative Questionnaire in a form supplied by the Administrative Agent.

“Affiliate” means, with respect to any Person, another Person that directly, or indirectly through one or more intermediaries, Controls or is Controlled by or is under common Control with the Person specified.

“Aggregate Commitments” means the Commitments of all the Lenders.

“Agreement” means this Credit Agreement, as amended, amended and restated, supplemented or otherwise modified from time to time.

“Applicable Foreign Obligor Documents” has the meaning specified in Section 5.21.

“Applicable Percentage” means with respect to any Lender at any time, the percentage (carried out to the ninth decimal place) of the Aggregate Commitments represented by such

Lender’s Commitment at such time. If the commitment of each Lender to make Loans and the obligation of the L/C Issuer to make L/C Credit Extensions have been terminated pursuant to Section 8.02 or if the Aggregate Commitments have expired, then the Applicable Percentage of each Lender shall be determined based on the Applicable Percentage of such Lender most recently in effect, giving effect to any subsequent assignments. The initial Applicable Percentage of each Lender is set forth opposite the name of such Lender on Schedule 2.01 or in the Assignment and Assumption pursuant to which such Lender becomes a party hereto, as applicable.

“Applicable Rate” means the following percentages per annum, based upon the Consolidated Leverage Ratio as set forth in the most recent Compliance Certificate received by the Administrative Agent pursuant to Section 6.02(b):

Any increase or decrease in the Applicable Rate resulting from a change in the Consolidated Leverage Ratio shall become effective as of the first Business Day immediately following the date a Compliance Certificate is delivered pursuant to Section 6.02(b); provided, however, that if a Compliance Certificate is not delivered when due in accordance with such Section, then Pricing Level III shall apply as of the first Business Day after the date on which such Compliance Certificate was required to have been delivered until such time such Compliance Certificate is delivered. The Applicable Rate in effect during the period from the Closing Date until the initial quarterly Compliance Certificate is delivered shall be determined based upon Pricing Level I.

“Approved Fund” means any Fund that is administered or managed by (a) a Lender, (b) an Affiliate of a Lender or (c) an entity or an Affiliate of an entity that administers or manages a Lender.

“Arranger” means Banc of America Securities, in its capacity as sole lead arranger and sole book manager.

“Assignee Group” means two or more Eligible Assignees that are Affiliates of one another or two or more Approved Funds managed by the same investment advisor.

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“Assignment and Assumption” means an assignment and assumption entered into by a Lender and an Eligible Assignee (with the consent of any party whose consent is required by Section 10.06(b)), and accepted by the Administrative Agent, in substantially the form of Exhibit E or any other form approved by the Administrative Agent.

“Attributable Indebtedness” means, on any date, (a) in respect of any Capitalized Lease of any Person, the capitalized amount thereof that would appear on a balance sheet of such Person prepared as of such date in accordance with GAAP, and (b) in respect of any Off-Balance Sheet Obligation, (i) in the case of an Off-Balance Sheet Obligation in an asset securitization transaction of the type described under clause (a) of the definition thereof, the unrecovered investment of transferees in transferred assets as to which such Person has or may have recourse obligations; or (ii) in the case of an Off-Balance Sheet Obligation in an off-balance sheet lease transaction of the type described under clauses (b), (c) and (d) of the definition thereof, the capitalized amount of the remaining lease payments under the relevant lease that would appear on a balance sheet of such Person prepared as of such date in accordance with GAAP if such lease were accounted for as a Capitalized Lease.

“Audited Financial Statements” means the audited consolidated balance sheets of the Parent and its Subsidiaries for the fiscal years ended December 31, 2002, December 31, 2003 and December 31, 2004, and the related consolidated statements of operations, shareholders’ equity and cash flows for each fiscal year of the Parent and its Subsidiaries, including the notes thereto.

“Availability Period” means the period from and including the Closing Date to the earliest of (a) the Maturity Date (b) the date of termination of the Aggregate Commitments pursuant to Section 2.06, and (c) the date of termination of the Commitment of each Lender to make Loans and of the obligation of the L/C Issuer to make L/C Credit Extensions pursuant to Section 8.02.

“Bank of America” means Bank of America, N.A. and its successors.

“Banc of America Securities” means Banc of America Securities LLC and its successors.

“Base Rate” means for any day a fluctuating rate per annum equal to the higher of (a) the Federal Funds Rate plus 1/2 of 1% and (b) the rate of interest in effect for such day as publicly announced from time to time by Bank of America as its “prime rate.” The “prime rate” is a rate set by Bank of America based upon various factors including Bank of America’s costs and desired return, general economic conditions and other factors, and is used as a reference point for pricing some loans, which may be priced at, above, or below such announced rate. Any change in such rate announced by Bank of America shall take effect at the opening of business on the day specified in the public announcement of such change.

“Base Rate Loan” means a Loan that bears interest at the Base Rate.

“Borrower” has the meaning specified in the introductory paragraph to this Agreement.

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“Borrowing” means a borrowing consisting of simultaneous Loans by the Borrower of the same Type and, in the case of Eurodollar Rate Loans having the same Interest Period, made by each of the Lenders pursuant to Section 2.01.

“Business Day” means any day other than a Saturday, Sunday or other day on which commercial banks are authorized to close under the Laws of, or are in fact closed in, the state where the Administrative Agent’s Office is located and, if such day relates to any Eurodollar Rate Loan, means any such day on which dealings in Dollar deposits are conducted by and between banks in the London interbank eurodollar market.

“Capitalized Lease” means, as applied to any Person, any lease of any property (whether real, personal or mixed) by that Person as lessee which in accordance with GAAP, is or should be accounted for, as a capital lease on the balance sheet of such Person.

“Cash Collateral” and “Cash Collateralize” has the meaning specified in Section 2.03(g).

“Cash Management Bank” means any party to a Cash Management Services Agreement with the Parent or any of its Subsidiaries which party is Bank of America, any Lender or any other Person approved by the Borrower and the Administrative Agent, and any Affiliate of any thereof.

“Cash Management Services Agreement” means any agreement to provide management services, including treasury, depository, overdraft, credit or debit card, electronic funds transfer and other cash management services that is entered into by and between a Loan Party and any Cash Management Bank.

“CHAMPUS” means the United States Department of Defense Civilian Health and Medical Program of the Uniformed Services, and its successor, the TriCare Management Activity.

“Change in Law” means the occurrence, after the date of this Agreement, of any of the following: (a) the adoption or taking effect of any law, rule, regulation or treaty, (b) any change in any law, rule, regulation or treaty or in the administration, interpretation or application thereof by any Governmental Authority or (c) the making or issuance of any request, guideline or directive (whether or not having the force of law) by any Governmental Authority.

“Change of Control” means, with respect to any Person, an event or series of events by which:

(a) any “person” or “group” (as such terms are used in Sections 13(d) and 14(d) of the Securities Exchange Act of 1934, but excluding any employee benefit plan of such person or its subsidiaries, and any person or entity acting in its capacity as trustee, agent or other fiduciary or administrator of any such plan) becomes the “beneficial owner” (as defined in Rules 13d-3 and 13d-5 under the Securities Exchange Act of 1934, except that a person or group shall be deemed to have “beneficial ownership” of all securities that such person or group has the right to acquire (such right, an “option right”), whether such right is exercisable immediately or only after the passage of time), directly or indirectly, of 33% or more of the equity securities of the Parent entitled to vote for members of the board of directors or equivalent governing body of the Parent on a fully-diluted basis (and taking into account all such securities that such person or group has the right to acquire pursuant to any option right); or

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(b) the Parent shall cease to own, directly or indirectly, 100% of GBIC on a fully diluted basis (and taking into account all such securities that any person or group has the right to acquire pursuant to any option right); or

(c) during any period of 12 consecutive months, a majority of the members of the board of directors or other equivalent governing body of the Parent ceases to be composed of individuals whose election or nomination to that board or equivalent governing body was approved by members of such board or equivalent body constituting at the time of such election or nomination at least 66 ²/3% of that board or equivalent governing body (excluding any individual whose initial nomination for, or assumption of office as, a member of that board or equivalent governing body occurs as a result of an actual or threatened solicitation of proxies or consents for the election or removal of one or more directors by any Person or group other than a solicitation for the election of one or more directors by or on behalf of the board of directors).

“Closing Date” means the first date all the conditions precedent in Section 4.01 are satisfied or waived in accordance with Section 4.01 (or, in the case of Section 4.01(b), waived by the Person entitled to receive the applicable payment).

“CMS” means the Centers for Medicare and Medicaid Services or any successor thereof.

“Code” means the Internal Revenue Code of 1986.

“Commitment” means, as to each Lender, its obligation to (a) make Loans to the Borrower pursuant to Section 2.01, and (b) purchase participations in L/C Obligations, in an aggregate principal amount at any one time outstanding not to exceed the amount set forth opposite such Lender’s name on Schedule 2.01 or in the Assignment and Assumption pursuant to which such Lender becomes a party hereto, as applicable, as such amount may be adjusted from time to time in accordance with this Agreement (including without limitation, pursuant to Section 2.14) and “Commitments” means the Commitments of all the Lenders.

“Company Materials” has the meaning specified in Section 6.02.

“Compliance Certificate” means a certificate substantially in the form of Exhibit D hereto.

“Consolidated EBITDA” means, for any period, for the Parent and its Subsidiaries determined on a consolidated basis, an amount equal to Consolidated Net Income for such period, plus (a) the following to the extent deducted in calculating such Consolidated Net Income: (i) Consolidated Interest Charges for such period; (ii) the provision for Federal, state, local and foreign income taxes for such period; (iii) depreciation and amortization expense for such period; (iv) non-cash stock-based employee compensation expense for such period; and (v) other expenses reducing such Consolidated Net Income which do not represent a cash item in such period or any future period and minus (b) the following to the extent included in calculating such Consolidated Net Income: (i) Federal, state, local and foreign income tax credits for such period and (ii) all non-cash items increasing Consolidated Net Income for such period.

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“Consolidated Funded Indebtedness” means, for the Parent and its Subsidiaries determined on a consolidated basis, as of any date of determination, without duplication, the sum of (a) the outstanding principal amount of all obligations, whether current or long-term, for borrowed money (including Obligations hereunder) and all obligations evidenced by bonds, debentures, notes, loan agreements or other similar instruments, (b) all purchase money Indebtedness (except as provided in clause (d) below), (c) all direct obligations arising under letters of credit (including standby and commercial), bankers’ acceptances, bank guaranties, surety bonds and similar instruments, (d) all obligations in respect of the deferred purchase price of property or services (other than trade accounts payable in the ordinary course of business and, except those being contested in good faith, not past due more than 60 days after the due date on which each such trade payable or account payable was created), (e) Attributable Indebtedness in respect of Capitalized Leases and Off-Balance Sheet Obligations, (f) without duplication, all Guarantees with respect to outstanding Indebtedness of the types specified in clauses (a) through (e) above of a Person other than the Borrower or any Subsidiary, and (g) all Indebtedness of the types referred to in clauses (a) through (f) above of any partnership or joint venture (other than a joint venture that is itself a corporation or limited liability company) in which Parent or a Subsidiary is a general partner or joint venturer, unless such Indebtedness is expressly made non-recourse to the Parent or any such Subsidiary.

“Consolidated Interest Charges” means, for any period, for the Parent and its Subsidiaries determined on a consolidated basis, the sum of (a) all interest, premium payments, debt discount, fees, charges and related expenses in connection with Indebtedness (including capitalized interest) or in connection with the deferred purchase price of assets, in each case to the extent treated as interest in accordance with GAAP, and (b) the portion of rent expense with respect to such period under Capitalized Leases that is treated as interest in accordance with GAAP.

“Consolidated Interest Coverage Ratio” means, as of any date of determination, the ratio of (a) Consolidated EBITDA for the period of the four prior fiscal quarters ending on such date to (b) Consolidated Interest Charges for such period.

“Consolidated Leverage Ratio” means, as of any date of determination, the ratio of (a) Consolidated Funded Indebtedness as of such date to (b) Consolidated EBITDA for the period of the four fiscal quarters most recently ended.

“Consolidated Net Income” means, for any period, for the Parent and the Subsidiaries on a consolidated basis in accordance with GAAP, the net income (excluding extraordinary gains and extraordinary losses) for that period.

“Consolidated Tangible Net Worth” means, as of any date of determination, for the Parent and its Subsidiaries on a consolidated basis, Shareholders’ Equity on that date minus Intangible Assets on that date.

“Consolidated Total Debt” means, as of any date of determination, Indebtedness of the Parent and its Subsidiaries, calculated on a consolidated basis in accordance with GAAP.

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“Consolidated Total Capitalization” means, as of any date of determination, the sum of (a) Consolidated Total Debt and (b) Shareholders’ Equity of the Parent and its Subsidiaries, calculated on a consolidated basis in accordance with GAAP.

“Consolidated Total Debt to Total Capitalization Ratio” means, the ratio at any fiscal quarter end of (a) Consolidated Total Debt to (b) Consolidated Total Capitalization.

“Contract Provider” means any person or any employee, agent or subcontractor of such Person who provides professional health care services under or pursuant to any contract with any Loan Party.

“Control” means the possession, directly or indirectly, of the power to direct or cause the direction of the management or policies of a Person, whether through the ability to exercise voting power, by contract or otherwise. “Controlling” and “Controlled” have meanings correlative thereto.

“Credit Extension” means each of the following: (a) a Borrowing and (b) an L/C Credit Extension.

“Debtor Relief Laws” means the Bankruptcy Code of the United States, and all other liquidation, conservatorship, bankruptcy, assignment for the benefit of creditors, moratorium, rearrangement, receivership, insolvency, reorganization, or similar debtor relief Laws of the United States, Ireland or other applicable jurisdictions from time to time in effect and affecting the rights of creditors generally.

“Default” means any event or condition that constitutes an Event of Default or that, with the giving of any notice, the passage of time, or both, would be an Event of Default.

“Default Rate” means an interest rate equal to (a) in the case of Eurodollar Rate Loans, the sum of (i) the Eurodollar Rate for such Loans plus (ii) the Applicable Rate applicable to such Loans, plus (iii) 2% per annum, (b) in the case of the Letter of Credit Fees, a rate equal to (i) the Applicable Rate plus (ii) 2% per annum and (c) in the case of Base Rate Loans and for all other Obligations, the sum of (i) the Base Rate for Base Rate Loans plus (ii) 2% per annum.

“Defaulting Lender” means any Lender that (a) has failed to fund any portion of a Borrowing or participations in L/C Obligations or participations in Swing Line Loans required to be funded by it hereunder within one Business Day of the date required to be funded by it hereunder, (b) has otherwise failed to pay over to the Administrative Agent or any other Lender any other amount required to be paid by it hereunder within one Business Day of the date when due, unless the subject of a good faith dispute, or (c) has been deemed insolvent or become the subject of a bankruptcy or insolvency proceeding.

“Disposition” or “Dispose” means the sale, transfer, license, lease or other disposition (including any sale and leaseback transaction) of any property by any Person, including any sale, assignment, transfer or other disposal, with or without recourse, of any notes issued by any other Person or accounts receivable or any rights and claims associated therewith or any capital stock of, or other Equity Interests in, any other Person; provided that the foregoing shall not be deemed to imply any such disposition is permitted under this Agreement.

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“Dollar” and “$” mean lawful money of the United States.

“Domestic Subsidiary” means any Subsidiary that is organized under the Laws of any political subdivision of the United States.

“Eligible Assignee” means (a) a Lender; (b) an Affiliate of a Lender; (c) an Approved Fund; and (d) any other Person (other than a natural person) approved by (i) the Administrative Agent, the L/C Issuer and the Swing Line Lender, and (ii) unless an Event of Default has occurred and is continuing, the Borrower (each such approval not to be unreasonably withheld or delayed); provided that notwithstanding the foregoing, “Eligible Assignee” shall not include the Borrower or any of its Affiliates or Subsidiaries.

“Environmental Laws” means any and all Federal, state, local, and foreign statutes, laws, regulations, ordinances, rules, judgments, orders, decrees, permits, concessions, grants, franchises, licenses, agreements or governmental restrictions relating to pollution and the protection of the environment or the release of any materials into the environment, including those related to hazardous substances or wastes, air emissions and discharges to waste or public systems.

“Environmental Liability” means any liability, contingent or otherwise (including any liability for damages, costs of environmental remediation, fines, penalties or indemnities), of any Loan Party or any of their respective Subsidiaries directly or indirectly resulting from or based upon (a) violation of any Environmental Law, (b) the generation, use, handling, transportation, storage, treatment or disposal of any Hazardous Materials, (c) exposure to any Hazardous Materials, (d) the release or threatened release of any Hazardous Materials into the environment or (e) any contract, agreement or other consensual arrangement pursuant to which liability is assumed or imposed with respect to any of the foregoing.

“Equity Interests” means, with respect to any Person, all of the shares of capital stock of (or other ownership or profit interests in) such Person, all of the warrants, options or other rights for the purchase or acquisition from such Person of shares of capital stock of (or other ownership or profit interests in) such Person, all of the securities convertible into or exchangeable for shares of capital stock of (or other ownership or profit interests in) such Person or warrants, rights or options for the purchase or acquisition from such Person of such shares (or such other interests), and all of the other ownership or profit interests in such Person (including partnership, member or trust interests therein), whether voting or nonvoting, and whether or not such shares, warrants, options, rights or other interests are outstanding on any date of determination.

“ERISA” means the Employee Retirement Income Security Act of 1974.

“ERISA Affiliate” means any trade or business (whether or not incorporated) under common control with the Parent within the meaning of Section 414(b) or (c) of the Code (and Sections 414(m) and (o) of the Code for purposes of provisions relating to Section 412 of the Code).

“ERISA Event” means (a) a Reportable Event with respect to a Pension Plan; (b) a withdrawal by the Parent or any ERISA Affiliate from a Pension Plan subject to Section 4063 of ERISA during a plan year in which it was a substantial employer (as defined in Section

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4001(a)(2) of ERISA) or a cessation of operations that is treated as such a withdrawal under Section 4062(e) of ERISA; (c) a complete or partial withdrawal by the Parent or any ERISA Affiliate from a Multiemployer Plan or notification that a Multiemployer Plan is in reorganization; (d) the filing of a notice of intent to terminate, the treatment of a Pension Plan amendment as a termination under Section 4041(c) of ERISA, or the commencement of proceedings by the PBGC to terminate a Pension Plan or Multiemployer Plan; (e) an event or condition which constitutes grounds under Section 4042 of ERISA for the termination of, or the appointment of a trustee to administer, any Pension Plan or Multiemployer Plan; or (f) the imposition of any liability under Title IV of ERISA, other than for PBGC premiums due but not delinquent under Section 4007 of ERISA, upon the Parent or any ERISA Affiliate.

“Eurodollar Rate” means, for any Interest Period with respect to a Eurodollar Rate Loan, the rate per annum equal to the British Bankers Association LIBOR Rate (“BBA LIBOR”), as published by Reuters (or other commercially available source providing quotations of BBA LIBOR as designated by the Administrative Agent from time to time) at approximately 11:00 a.m., London time, two Business Days prior to the commencement of such Interest Period, for Dollar deposits (for delivery on the first day of such Interest Period) with a term equivalent to such Interest Period. If such rate is not available at such time for any reason, then the “Eurodollar Rate” for such Interest Period shall be the rate per annum determined by the Administrative Agent to be the rate at which deposits in Dollars for delivery on the first day of such Interest Period in same day funds in the approximate amount of the Eurodollar Rate Loan being made, continued or converted by Bank of America and with a term equivalent to such Interest Period would be offered by Bank of America’s London Branch to major banks in the London interbank eurodollar market at their request at approximately 11:00 a.m. (London time) two Business Days prior to the commencement of such Interest Period.

“Eurodollar Rate Loan” means a Loan that bears interest at the Eurodollar Rate.

“Event of Default” has the meaning specified in Section 8.01.

“Excluded Taxes” means, with respect to the Administrative Agent, any Lender, the L/C Issuer or any other recipient of any payment to be made by or on account of any obligation of any Loan Party under any of the Loan Documents, (a) taxes imposed on or measured by its overall net income (however denominated), and franchise taxes imposed on it (in lieu of net income taxes), as a result of a present or former connection between the Administrative Agent, such Lender or the L/C Issuer (or any other recipient of any payment to be made by or on account of any obligation of any Loan Party under any of the Loan Documents) and the jurisdiction of the Governmental Authority imposing such tax or any political subdivision or taxing authority thereof or therein (other than any such connection arising solely from the Administrative Agent, Lender or L/C Issuer (or any other recipient of any payment to be made by or on account of any obligation of any Loan Party under any of the Loan Documents) having executed, delivered or performed its obligations or received a payment under, or enforced, this Agreement or any other Loan Document), (b) any branch profits taxes imposed by the United States or any similar tax imposed by any other jurisdiction in which such Loan Party is located and (c) in the case of a Foreign Lender (other than a Foreign Lender who becomes a Lender as a result of an assignment to such Lender pursuant to a request by the Borrower under Section 10.13), any withholding tax that is imposed on amounts payable to such Foreign Lender at the

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