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                                                                   Exhibit 10.18

[ACTIVBIOTICS(R) LOGO]

January 17, 2005

Dr. Joshua Boger
Chairman & Chief Executive Officer
Vertex Pharmaceuticals, Inc
130 Waverly Street
Cambridge, MA 02139

Dear Dr. Boger:

On behalf of the Board of Directors of ActivBiotics (the "Company"), I am
pleased to offer you a position as a member of the Board of Directors, effective
December 30, 2005.

As an independent member of the Board, we offer you the following compensation
package:

CASH COMPENSATION: $10,000 annual compensation, payable in 4 quarterly
installments.

STOCK OPTIONS: You will be eligible to receive periodic grants of stock options,
as recommended by the ActivBiotics Board of Directors Compensation Committee,
(once this committee is newly constituted) and upon approval by the full Board
of Directors. These option grants will vest in equal monthly installments over a
four-year period from the date of grant, so long as you continue to serve as a
Director, at an exercise price equal to the Fair market value of the Company's
Common Stock on the date of the grant.

EXPENSES: The Company will reimburse you for all reasonable and necessary
out-of-pocket expenses incurred by you in connection with the performance of
your duties as a Director.

INDEMNIFICATION: The Company shall, to the fullest extent permitted Delaware
General Corporation law as then in effect, indemnify you if as a result of your
position as a director of the Company you are involved in any manner (including,
without limitation, as a party or a witness) or are threatened to be made so
involved in any threatened, pending or completed investigation, claim, action,
suit or proceeding, whether civil, criminal, administrative or investigative
(including, without limitation, any action, suit or proceeding by or in the
right of the Company to procure a judgment in its favor) (a "Proceeding")
against all expenses (including reasonable attorneys' fees), judgments, fines
and amounts paid in settlement actually and reasonably incurred by you in
connection with such Proceeding Such indemnification shall be a contract right

     ActivBiotics, Inc. 110 Hartwell Avenue, Lexington, MA02421 781-372-1800
                      Fax 781-274-8638 www.activbiotics.com

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and shall include the right to receive payment in advance of any expenses
incurred by you in connection with such Proceeding.

INSURANCE. The Company is covered by Directors and Officers Liability and
Reimbursement Insurance issued by Carolina Casualty Company with a $5,000,000
liability limit.

Josh, ActivBiotics has an exciting future but of course faces many challenges in
realizing its ultimate potential. We can only achieve our vision for the Company
with the help individuals like you who have a significant breath and depth of
experience, demonstrated talent and personal commitment. To that end I welcome
you to join the ActivBiotics Board of Directors.

Sincerely,


/s/ Alan Dunton, MD
-----------------------------
Dr. Alan Dunton, MD
Chairman, ActivBiotics

I hereby accept my election as a member of the Board of Directors of
ActivBiotics; effective December 30, 2005.


/s/ Joshua Boger
-----------------------------
Joshua Boger


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