Infection

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] An infectious disease, also known as a transmissible disease or communicable disease, is an illness resulting from an infection.

Infections can be caused by a wide range of pathogens, most prominently bacteria and viruses.^[2] Hosts can fight infections using their immune system. Mammalian hosts react to infections with an innate response, often involving inflammation, followed by an adaptive response.

Specific medications used to treat infections include antibiotics, antivirals, antifungals, antiprotozoals, and antihelminthics. Infectious diseases resulted in 9.2 million deaths in 2013 (about 17% of all deaths).^[3] The branch of medicine that focuses on infections is referred to as infectious disease.^[4]

The symptoms of an infection depend on the type of disease. Some signs of infection affect the whole body generally, such as fatigue, loss of appetite, weight loss, fevers, night sweats, chills, aches and pains. Others are specific to individual body parts, such as skin rashes, coughing, or a runny nose.^{[citation needed]}

In certain cases, infectious diseases may be asymptomatic for much or even all of their course in a given host. In the latter case, the disease may only be defined as a "disease" (which by definition means an illness) in hosts who secondarily become ill after contact with an asymptomatic carrier. An infection is not synonymous with an infectious disease, as some infections do not cause illness in a host.^[9]

As bacterial and viral infections can both cause the same kinds of symptoms, it can be difficult to distinguish which is the cause of a specific infection.^[10] Distinguishing the two is important, since viral infections cannot be cured by antibiotics whereas bacterial infections can.^[11]

Infection of an ingrown toenail; there is pus (yellow) and resultant inflammation (redness and swelling around the nail).

^[16][17][18] This image depicts the steps of pathogenic infection.

A southern house mosquito (Culex quinquefasciatus) is a vector that transmits the pathogens that cause West Nile fever and avian malaria among others.

Four nutrient agar plates growing colonies of common Gram negative bacteria.

Washing one's hands, a form of hygiene, is an effective way to prevent the spread of infectious disease.^[44]

Mary Mallon (a.k.a. Typhoid Mary) was an asymptomatic carrier of typhoid fever. Over the course of her career as a cook, she infected 53 people, three of whom died.

Deaths due to infectious and parasitic diseases per million persons in 2012

  28–81

  82–114

  115–171

  172–212

  213–283

  284–516

  517–1,193

  1,194–2,476

  2,477–3,954

  3,955–6,812

Disability-adjusted life year for infectious and parasitic diseases per 100,000 inhabitants in 2004.^[55]

  no data

  ≤250

  250–500

  500–1000

  1000–2000

  2000–3000

  3000–4000

  4000–5000

  5000–6250

  6250–12,500

  12,500–25,000

  25,000–50,000

  ≥50,000

Great Plague of Marseille in 1720 killed 100,000 people in the city and the surrounding provinces

East German postage stamps depicting four antique microscopes. Advancements in microscopy were essential to the early study of infectious diseases.

Herrerasaurus skull.