Immune states in long-term space flights.

Maintenance of health, as contrasted to illness induced by infectious agents, reflects a tenuous balance between the host's resistance and the number and genetic characteristics of the infectious organisms present. Nature has evolved both nonspecific defense mechanisms and specific immune systems to protect the body against invasion by exogenous organisms or the numerous agents which may reside dormant within the host. Long-term space flights with their accompanying prolonged weightlessness, unaccustomed environmental factors, emotional disturbances, and unforeseen influences may alter the host's natural or specific immune states. The non-specific and specific host defenses will be discussed, and the particular effects which their alteration might have on provocation of latent viral infections will be considered. Viruses which classically may be induced to cause recurrent infections such as herpes simplex and herpes zoster will be described, but in addition the effect that altered host defenses might have on slow virus infections such as kuru and virus-induced malignancies will be emphasized.