Spaceflight results in immunosuppression which is likely due mainly to neurohumoral factors released in response to intermittent stress effects during flight. However, no major non-physiological health problems have been reported during or following spaceflight, but diseases resulting from immunosuppression could occur on long-duration missions and would include bacterial, fungal, and viral infections in addition to increased incidence of neoplasia and autoimmunity. Pharmacokinetics and pharmacodynamics appear to be altered during spaceflight and, as a consequence, alternative drug administration and dosing procedures will need to be developed. Moderate exercise training enhances immune function, but in-flight exercise may affect immunological parameters and immunity in ways not yet ascertained. Hyperosmolality may enhance some immune parameters, and attenuate others especially when associated with dehydration and exercise. Reducing in-flight stress may attenuate flight-induced immunosuppression, but pharmacological interventions may be essential to prevent undesirable immune responses which may occur on long-duration missions to Mars.