The immune system is responsible for fighting foreign bacteria, viruses, etc. To work properly, it must be able to detect a wide variety of invaders, yet still distinguish between foreign particles and body tissue cells. In space, the capability of the immune system is greatly diminished. T-Cells, a type of lymphocyte (itself a type of white blood cell), do not reproduce properly, and those that are produced are less capable of fighting off infections. In addition, some viruses become more active in space, becoming a serious threat for a weakened immune system. Combined with the close-quarters and lowered hygiene, this immunodeficiency could become a major problem for long term missions.
Current solutions include an array of vaccinations, irradiation of food items, and sterilization of the spacecraft. In addition, a vast selection of supplements are administered in an effort to prevent immunodeficiency. Immunological parameters (e.g. lymphocyte count) are monitored in all individuals, so as to ensure a healthy and functioning immune system. When deficiencies are detected, personalized countermeasures are created so as to right the imbalance.