to new exposures, viruses already present in the body—which would normally be suppressed—become active. in space, t-cells do not reproduce properly, and the cells that do exist are less able to fight off infection. nasa research is measuring the change in the immune systems of its astronauts as well as performing experiments with t-cells in space. on april 29, 2013, scientists in rensselaer polytechnic institute, funded by nasa, reported that, during spaceflight on the international space station, microbes seem to adapt to the space environment in ways "not observed on earth" and in ways that "can lead to increases in growth and virulence". in march 2019, nasa reported that latent viruses in humans may be activated during space missions, adding possibly more risk to astronauts in future deep-space missions. increased infection risk a 2006 space shuttle experiment found that salmonella typhimurium, a bacterium that can cause food poisoning, became more virulent when cultivated in space. on april 29, 2013, scientists in rensselaer polytechnic institute, funded by nasa, reported that, during spaceflight on the international space station, microbes seem to adapt to the space environment in ways "not observed on earth" and in ways that "can lead to increases in growth and virulence". more recently, in 2017, bacteria were found to be more resistant to antibiotics and to thrive in the near-weightlessness of space. microorganisms have been observed to survive the vacuum of outer space. researchers in 2018 reported, after detecting the presence on the international space station (iss) of five enterobacter bugandensis bacterial strains, none pathogenic to humans, that microorganisms on iss should be carefully monitored to continue assuring a medically healthy environment for astronauts. effects of fatigue further information: fatigue and sleep loss during spaceflight human spaceflight often requires astronaut crews to endure long periods without rest. studies have shown that lack of sleep can cause fatigue that leads to errors while performing critical tasks. also, individuals who are fatigued often cannot determine the degree of their impairment. astronauts and ground crews frequently suffer from the effects of sleep deprivation and circadian rhythm disruption. fatigue due to sleep loss, sleep shifting and work overload could cause performance errors that put space flight participants at risk of compromising mission objectives as well as the health and safety of those on board. loss of balance leaving and returning to earth's gravity causes “space sickness,” dizziness, and loss of balance in astronauts. by studying how changes can affect balance in the human body—involving the senses, the brain, the inner ear, and blood pressure—nasa hopes to develop treatments that can be used on earth and in space to correct balance disorders. until then, nasa's astronauts must rely on a medication called midodrine (an “anti-dizzy” pill that temporarily increases blood pressure), and/or promethazine to help carry out the tasks they need to do to return home safely. loss of bone density spaceflight osteopenia is the bone loss associated with human spaceflight. the metabolism of calcium is limited in microgravity and will cause calcium to leak out of bones. after a 3–4 month trip into space, it takes about 2–3 years to regain lost bone density. new techniques are being developed to help astronauts recover faster. research in the following areas holds the potential to aid the process of growing new bone: diet and exercise changes may reduce osteoporosis. vibration therapy may stimulate bone growth. medication could trigger the body to produce more of the protein responsible for bone growth and formation. loss of muscle mass main article: reduced muscle mass, strength and performance in space in space, muscles in the legs, back, spine, and heart weaken and waste away because they no longer are needed to overcome gravity, just as people lose muscle when they age due to reduced physical activity. astronauts rely on research in the following areas to build muscle and maintain body mass: exercise may build muscle if at least two hours a day is spent doing resistance training routines. hormone supplements (hgh) may be a way to tap into the body's natural growth signals. medication may trigger the body into producing muscle growth proteins. neuromuscular electrical stimulation as a method to prevent muscle atrophy. impairment of eyesight main article: visual impairment due to intracranial pressure after long space flight missions, astronauts may experience severe eyesight problems. such vision problems may be a major concern for future deep space flight missions, including a human mission to mars. loss of mental abilities and risk of alzheimer's disease