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.[1][2] An infectious disease, also known as a transmissible disease or communicable disease, is an illness resulting from an infection. Infections are caused by infectious agents (pathogens) including: Viruses and related agents such as viroids and prions Bacteria Fungi, further subclassified into: Ascomycota, including yeasts such as Candida, filamentous fungi such as Aspergillus, Pneumocystis species, and dermatophytes, a group of organisms causing infection of skin and other superficial structures in humans.

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 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.