Latency is a characteristic exhibited by certain infectious diseases where the bacteria or virus stops replicating and hides in a part of the body that is isolated from the immune system, such as a cyst or the reproductive system, or even inside individual intact cells. During this period of time, the patient is often without symptoms of the disease. However, diseases with latency periods are often difficult to treat because the drugs to treat it cannot pass the barrier from the bloodstream to where the disease is effectively "hiding".
In Poison, House treats a women in her 80s who is suffering from neurosyphilis although she was treated for the disease when she contracted it in the 1930s before the development of antibiotic treatments for the disease. The disease had therefore been latent for over 70 years.
See also incubation period, the time between exposure to a contagious infection and the time the patient exhibits symptoms.