Eligibility Details:  
        -  INCLUSION CRITERIA:
        All participants (vaccinees, household contacts, and intimate contacts) must meet all of
        the following criteria:
          1. Age 18 to 49 years for vaccinees. Age 18 to 65 years for household and intimate
             contacts.
          2. Negative FDA-approved HIV test.
          3. Available and willing to participate in follow-up visits and tests for the duration of
             the study.
          4. Willing to have samples stored for future research.
        The following inclusion criteria apply to vaccinees and intimate contacts, but not to
        household contacts:
          1. In good general health without clinically significant medical history.
          2. Willing to discuss HIV infection risks with the study clinicians, assessed as low risk
             for HIV infection, amenable to HIV risk reduction counseling, and committed to
             maintaining behavior consistent with low risk of HIV exposure through the last
             required clinic visit in the protocol schedule.
          3. Negative beta-HCG pregnancy test for females presumed to be of reproductive potential.
          4. Female vaccinees and male intimate contacts must meet one of the following criteria:
             -The female vaccinee has no reproductive potential because of menopause (1 year
             without menses) or because of a hysterectomy, bilateral oophorectomy,
             medically-documented ovarian failure, or tubal ligation.
             or
             -The female vaccinee and her male intimate contact(s) agree to be heterosexually
             inactive or consistently practice contraception at least 21 days prior to each
             vaccination through 28 days following each vaccination. Acceptable methods of
             contraception include any of
             the following:
               -  condoms, male or female, with a spermicide.
               -  diaphragm or cervical cap with spermicide.
               -  contraceptive pills, Norplant, or Depo-Provera (and is not on any medications
                  that would interfere with the effectiveness of these contraceptive agents).
               -  male partner has previously undergone a vasectomy for which there is
                  documentation.
               -  intrauterine device.
          5. Male vaccinees and female intimate contacts must agree to consistently practice
             abstinence or effective birth control (described above) and for 28 days following each
             vaccination.
        The following inclusion criteria apply only to vaccinees and not to household or intimate
        contacts:
          1. Willing to receive HIV test results and abide by NIH guidelines for partner
             notification of positive HIV results.
          2. Physical examination and laboratory results without clinically significant findings
             within the 8 weeks prior to enrollment.
          3. Willing to avoid other investigational and/or HIV vaccinations, other than the study
             agent, from screening through the end-of-study visit.
          4. Safety laboratory criteria within 8 weeks prior to enrollment:
               -  Hematopoietic:
                    -  White blood cell count and lymphocyte count within 25% of both the lower
                       limit and upper limit of normal for the NIH CC (ranges: 2.985-12.55 K/uL for
                       white blood cells and 0.885 - 4.675 K/uL for lymphocytes).
                    -  Platelet count of least 150,000/mm3.
                    -  Hemoglobin count within 10% of both the lower limit and upper limit of
                       normal for the NIH CC (ranges: Females 10.08 - 17.27 g/dL, Males 12.33
                       -19.25 g/dL)
               -  Renal: Blood urea nitrogen (BUN) <23 mg/dL; creatinine within normal limits for
                  the NIH CC (females: 0.51-0.95 mg/dL; males: 0.67-1.17 mg/dL).
               -  Hepatic: Serum direct bilirubin within normal limits for the NIH CC (0.0 to 0.3
                  mg/dL).
               -  Metabolic: Alanine aminotransferase (ALT) < 2 times upper limit of normal range
                  (females: <66 U/L; males: <82 U/L).
               -  Endocrine: Serum glucose within normal range.
          5. Additional laboratory criteria:
               -  Immunologic: No history of hypogammaglobulinemia.
               -  Serologic: Ad4 neutralizing antibody 80% inhibitory dilution <1:100. (This
                  criterion does not apply to participants in Arm B.)
          6. Willing to follow precautions for preventing the spread of adenovirus in the
             community.
          7. Males must agree not to donate sperm for 28 days following each study vaccination.
        EXCLUSION CRITERIA:
        A participant (vaccinees, household contacts, and intimate contacts) will be excluded if
        they have the following:
        1. Any condition that, in the investigator s judgement, places the subject at undue risk by
        participating in the study.
        The following exclusion criteria apply to vaccinees and intimate contacts, but not to
        household contacts:
          1. History of any prior disease or therapy which would affect immune or pulmonary
             function.
          2. Prior malignancy, except curatively-treated basal cell or squamous cell carcinoma of
             the skin or carcinoma in situ of the cervix.
          3. History of radiation therapy or cytotoxic/cancer chemotherapy.
          4. History of diabetes mellitus.
          5. Immunodeficiency or autoimmune disease.
          6. Acute infection or a recent history (within 6 months) of chronic infection suggestive
             of immunodeficiency.
          7. Taking any glucocorticoids or other immunosuppressive medications.
          8. Chronic respiratory disorders including asthma, emphysema, interstitial lung disease,
             pulmonary hypertension, recurrent pneumonia, or recent or ongoing respiratory tract
             infection. If a respiratory disorder is transient, the study vaccination may be
             deferred without excluding the participant.
          9. Female of childbearing potential who is breast-feeding or planning pregnancy during
             the period from enrollment through 28 days following the last study vaccination.
        The following exclusion criteria apply only to vaccinees and not to household or intimate
        contacts:
          1. Any medical, psychiatric, or social condition, or occupational or other responsibility
             that, in the judgment of the investigator, would interfere with or serve as a
             contraindication to receipt of live virus vaccine, protocol adherence, or a
             participant s ability to give informed consent.
          2. Psychiatric condition that precludes compliance with the protocol; past or present
             psychoses; past or present bipolar disorder requiring therapy that has not been well
             controlled on medication for the past two years; disorder requiring lithium; or
             suicidal ideation occurring within five years prior to enrollment.
          3. Individuals that live in the same house or apartment with any of the following will be
             excluded:
               1. An individual under 18 years of age.
               2. An elderly individual (>65 years of age).
               3. An immunocompromised or immunosuppressed individual.
               4. An individual with chronic respiratory disease.
               5. A woman who is currently pregnant or breastfeeding or planning a pregnancy during
                  the period of vaccination.
          4. Healthcare worker who has direct contact with immunodeficient, unstable, elderly, or
             pediatric patients.
          5. Individuals caring for children <18 years of age or elderly individuals.
          6. Receipt of any of the following:
               -  Antiviral medications within 30 days prior to vaccination.
               -  Blood products within 120 days prior to HIV screening.
               -  Immunoglobulin within 60 days prior to HIV screening.
               -  Investigational research drugs or any other investigational agent that in the
                  judgement of the Principal Investigator might interact with the study vaccine
                  within 30 days prior to initial study vaccine administration.
               -  Allergy treatment with antigen injections within 30 days of study vaccine
                  administration.
          7. Active hepatitis B or C infection (i.e., hepatitis B or C positive serology with the
             presence of virus antigen or DNA). Ongoing viral replication will be confirmed by a
             hepatitis B antigen test
             or hepatitis C viral load.
          8. History of Guillain-Barr(SqrRoot)(Copyright) syndrome.
          9. Indeterminate HIV Western blot test.
         10. Prior receipt of the Merck Ad5-based HIV vaccine.
         11. Refusal of any of the individual s intimate contacts to enroll as such on this study.