Disease the time between infection and the appearance of symptoms. The latency period is the time between infection and the ability of the disease to spread to another person, which may precede, follow, or be simultaneous with the appearance of symptoms. Some viruses also exhibit a dormant phase, called viral latency, in which the virus hides in the body in an inactive state. For example, varicella zoster virus causes chickenpox in the acute phase; after recovery from chickenpox, the virus may remain dormant in nerve cells for many years, and later cause herpes zoster (shingles). Diseases may be classified by cause,
Contagious disease illustrated classically by tuberculosis, which is clearly transmissible from person to person, but was not classically considered a contagious disease. In the present day, most sexually transmitted diseases are considered contagious, but only some of them are subject to medical isolation. Originally, the term referred to a "contagion" (a derivative of 'contact') or disease transmissible only by direct physical contact. In the modern day, the term has sometimes been broadened to encompass "any" communicable or infectious disease. Often the word can only be understood in context, where it is used to emphasise very infectious, easily transmitted, or especially severe communicable