Physical exercise may enhance some and depress other immune functions. The biological importance of these changes is not fully elucidated. Acute endurance exercise results in a relatively large redistribution of leukocytes between circulating blood and other tissues, as well as an increase in circulating cytokines. Some of these changes have been related to energy metabolism. A temporal correlation has been observed between altered immune functions and resistance to infections. A post-exercise infection can be either the result of a pre-exercise, sub-clinical infection amplified by the performed work or a novel infection, acquired during a period of decreased immune function shortly after exercise. Animal experiments have demonstrated that the susceptibility to infections after exercise depends on exercise intensity and duration, type of pathogen and time of inoculation. Exercise before inoculation with some bacterial agents can enhance resistance to infection, while exercise during an ongoing viral or bacterial infection worsens symptoms and enhances the risk for complications. Most studies demonstrate a deleterious effect of physical exercise in conjunction with infectious episodes.