The race between infection and immunity: how do pathogens set the pace?

Infection is often referred to as a race between pathogen and immune response. This metaphor suggests that slower growing pathogens should be more easily controlled. However, a growing body of evidence shows that many chronic infections are caused by failure to control slow growing pathogens. The slow growth of pathogens seems to directly affect the kinetics of the immune response. Compared with the response to fast growing pathogens, the T-cell response to slow pathogens is delayed in its initiation, lymphocyte expansion is slow and the response often fails to clear the pathogen, leading to chronic infection. Understanding the 'rules of the race' for slow growing pathogens has important implications for vaccine design and immune control of many chronic infections.