Viruses have been known to remain dormant in some people over time. In some cases, people may go for years while playing host to a virus which they didn't know they had. The idea is that serene environments with little stress may in most cases limit the occurrence of a virus. However, the idea of a tranquil set of conditions fades away when astronauts head to space.
In addition to the "bumpy" journey, astronauts have to undergo physical stress. This includes massive G force upon entry and exit of earth's atmosphere, zero gravity, changes in sleeping patterns and cosmic radiation. This is in addition to the psychological and emotional toll associated with separation from loved ones. This eventually culminates in the astronauts having intense periods of stress. As has been noted above, such periods are the reason behind the shedding of viruses (shedding is the resurfacing of a previously dormant illness or virus). Viruses such as chicken pox and shingles would generally shed in space.
It is, however, the case that a different virus is shedding in space: the herpes virus.
A recent study by Frontiers in Microbiology has shown the world the sheer stress associated with space in a different light. The report revealed the fact that the herpes virus has shed in more than half of the astronauts who have traveled to space. Researchers have attributed this awakening of dormant viruses to the stress associated with the journey to and from space. Satish K. Mehta, one of the study’s lead authors, states that “…47 of the 89 astronauts on short space flights (53%) and 14 of the 23 astronauts on long space flights (61%) shed the herpes virus in their saliva and urine samples.”
Nevertheless, the report also states that the shedding of the virus was not necessarily a sign of recurrence of virus symptoms. As was noted, of all the astronauts, “…only six showed symptoms of reactivation of the virus and all symptoms were minor.” Despite this, given the infectious nature of these viruses, they warned that the risk of infecting others especially shortly upon the return to earth was quite high. For astronauts heading to space for longer periods, the risk of shedding keeps increasing and so does the aforementioned infection risk. Vaccination of astronauts is thus the countermeasure being sought, especially for diseases such as chicken pox, which can have dire consequences on one should it shed and resurge in space.