Prolonged human spaceflight to another planet or an asteroid will introduce unique challenges of mitigating the risk of infection. During space travel, exposure to microgravity, radiation, and stress alter human immunoregulatory responses, which can in turn impact an astronaut's ability to prevent acquisition of infectious agents or reactivation of latent infection. In addition, microgravity affects virulence, growth kinetics, and biofilm formation of potential microbial pathogens. These interactions occur in a confined space in microgravity, providing ample opportunity for heavy microbial contamination of the environment. In addition, there is the persistence of aerosolized, microbe-containing particles. Any mission involving prolonged human spaceflight must be carefully planned to minimize vulnerabilities and maximize the likelihood of success.