"Infectious disease" redirects here. For the medical specialty, see infectious disease (medical specialty). For the journal, see Infectious Diseases (journal).

An infection is the invasion of an organism's body tissues by disease-causing agents, their multiplication, and the reaction of host tissues to the infectious agents and the toxins they produce. An infectious disease, also known as a transmissible disease or communicable disease, is an illness resulting from an infection.

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..

Among the many varieties of microorganisms, relatively few cause disease in otherwise healthy individuals. Infectious disease results from the interplay between those few pathogens and the defenses of the hosts they infect. The appearance and severity of disease resulting from any pathogen depend upon the ability of that pathogen to damage the host as well as the ability of the host to resist the pathogen. However, a host's immune system can also cause damage to the host itself in an attempt to control the infection. Clinicians, therefore, classify infectious microorganisms or microbes according to the status of host defenses - either as primary pathogens or as opportunistic pathogens.

 Primary pathogens cause disease as a result of their presence or activity within the normal, healthy host, and their intrinsic virulence (the severity of the disease they cause) is, in part, a necessary consequence of their need to reproduce and spread. Many of the most common primary pathogens of humans only infect humans, however, many serious diseases are caused by organisms acquired from the environment or that infect non-human hosts.

Regards

Jessica Rose

Journal of Clinical Infectious Diseases & Practice

E-mail: editor.jcidp@emedsci.com