Symptomatic infections are apparent and clinical, whereas an infection that is active but does not produce noticeable symptoms may be called inapparent, silent, subclinical, or occult. An infection that is inactive or dormant is called a latent infection. An example of a latent bacterial infection is latent tuberculosis. Some viral infections can also be latent, examples of latent viral infections are any of those from the Herpesviridae family. The word infection can denote any presence of a particular pathogen at all (no matter how little) but also is often used in a sense implying a clinically apparent infection (in other words, a case of infectious disease). This fact occasionally creates some ambiguity or prompts some usage discussion; to get around this it is common for health professionals to speak of colonization (rather than infection) when they mean that some of the pathogens are present but that no clinically apparent infection (no disease) is present. Different terms are used to describe infections. The first is an acute infection. An acute infection is one in which symptoms develop rapidly; its course can either be rapid or protracted. The next is a chronic infection. A chronic infection is when symptoms develop gradually, over weeks or months, and are slow to resolve. A subacute infection is one in which symptoms take longer to develop than in an acute infection but arise more quickly than a chronic infection. A latent infection is a type of infection that may occur after an acute episode; the organism is present but symptoms are not; after time the disease can reappear. A focal infection is defined as the initial site of infection from which organisms travel via the bloodstream to another area of the body.

Opportunistic pathogens can cause an infectious disease in a host with depressed resistance (immunodeficiency) or if they have unusual access to the inside of the body (for example, via trauma). Opportunistic infection may be caused by microbes ordinarily in contact with the host, such as pathogenic bacteria or fungi in the gastrointestinal or the upper respiratory tract, and they may also result from (otherwise innocuous) microbes acquired from other hosts (as in Clostridium difficile colitis) or from the environment as a result of traumatic introduction (as in surgical wound infections or compound fractures). An opportunistic disease requires impairment of host defenses, which may occur as a result of genetic defects (such as Chronic granulomatous disease), exposure to antimicrobial drugs or immunosuppressive chemicals (as might occur following poisoning or cancer chemotherapy), exposure to ionizing radiation, or as a result of an infectious disease with immunosuppressive activity (such as with measles, malaria or HIV disease). Primary pathogens may also cause more severe disease in a host with depressed resistance than would normally occur in an immunosufficient host.

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