<Dd> A primary infection is infection that is, or can practically be viewed as, the root cause of the current health problem . In contrast, a secondary infection is a sequela or complication of a root cause . For example, pulmonary tuberculosis is often a primary infection, but an infection that happened only because a burn or penetrating trauma (the root cause) allowed unusual access to deep tissues is a secondary infection . Primary pathogens often cause primary infection and also often cause secondary infection . Usually opportunistic infections are viewed as secondary infections (because immunodeficiency or injury was the predisposing factor). </Dd> <P> One way of proving that a given disease is "infectious", is to satisfy Koch's postulates (first proposed by Robert Koch), which demands that the infectious agent be identified only in patients and not in healthy controls, and that patients who contract the agent also develop the disease . These postulates were first used in the discovery that Mycobacteria species cause tuberculosis . Koch's postulates cannot be applied ethically for many human diseases because they require experimental infection of a healthy individual with a pathogen produced as a pure culture . Often, even clearly infectious diseases do not meet the infectious criteria . For example, Treponema pallidum, the causative spirochete of syphilis, cannot be cultured in vitro--however the organism can be cultured in rabbit testes . It is less clear that a pure culture comes from an animal source serving as host than it is when derived from microbes derived from plate culture . Epidemiology is another important tool used to study disease in a population . For infectious diseases it helps to determine if a disease outbreak is sporadic (occasional occurrence), endemic (regular cases often occurring in a region), epidemic (an unusually high number of cases in a region), or pandemic (a global epidemic). </P> <P> Infectious diseases are sometimes called contagious disease when they are easily transmitted by contact with an ill person or their secretions (e.g., influenza). Thus, a contagious disease is a subset of infectious disease that is especially infective or easily transmitted . Other types of infectious / transmissible / communicable diseases with more specialized routes of infection, such as vector transmission or sexual transmission, are usually not regarded as "contagious", and often do not require medical isolation (sometimes loosely called quarantine) of victims . However, this specialized connotation of the word "contagious" and "contagious disease" (easy transmissibility) is not always respected in popular use . Infectious diseases are commonly transmitted from person to person through direct contact . The types of contact are through person to person and droplet spread . Indirect contact such as airborne transmission, contaminated objects, food and drinking water, animal person contact, animal reservoirs, insect bites, and environmental reservoirs are another way infectious diseases are transmitted, </P> <P> Infections can be classified by the anatomic location or organ system infected, including: </P>

The person to whom the infectious agent is passed is