Infectious disease

For other uses, see Infection (disambiguation).
"Infectious disease" redirects here. For the medical specialty, see Infectious diseases (medical specialty). For the journal, see Infectious Disease (journal).

Infection
False-colored electron micrograph showing a malaria sporozoite migrating through the midgut epithelium of a rat
Specialty	Infectious diseases
Causes	bacterial, viral, parasitic, fungal, prion

An infection is the invasion of tissues by pathogens, their multiplication, and the reaction of host tissues to the infectious agent 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,^[3] and antihelminthics. Infectious diseases resulted in 9.2 million deaths in 2013 (about 17% of all deaths).^[4] The branch of medicine that focuses on infections is referred to as infectious disease.^[5]


Contents

• 
  1Types
  
• 
  2Signs and symptoms
  
  • 
    2.1Bacterial or viral
    
• 
  3Pathophysiology
  
  • 
    3.1Colonization
    
  • 
    3.2Disease
    
  • 
    3.3Transmission
    
• 
  4Diagnosis
  
  • 
    4.1Symptomatic diagnostics
    
  • 
    4.2Microbial culture
    
  • 
    4.3Microscopy
    
  • 
    4.4Biochemical tests
    
  • 
    4.5PCR-based diagnostics
    
  • 
    4.6Metagenomic sequencing
    
  • 
    4.7Indication of tests
    
  • 
    4.8Classification
    
• 
  5Prevention
  
  • 
    5.1Immunity
    
• 
  6Treatments
  
• 
  7Epidemiology
  
  • 
    7.1Historic pandemics
    
  • 
    7.2Emerging diseases
    
• 
  8Germ theory of disease
  
  • 
    8.1Medical specialists
    
• 
  9Society and culture
  
• 
  10Fossil record
  
• 
  11Outer space
  
• 
  12See also
  
• 
  13References
  
• 
  14External links
  

Types[edit]
Infections are caused by infectious agents (pathogens) including:

• 
  Bacteria (e.g. Mycobacterium tuberculosis, Staphylococcus aureus, Escherichia coli, Clostridium botulinum, and Salmonella spp.)
  
• 
  Viruses and related agents such as viroids. (E.g. HIV, Rhinovirus, Lyssaviruses such as Rabies virus, Ebolavirus and Severe acute respiratory syndrome coronavirus 2)
  
• 
  Fungi, further subclassified into:
  
  • 
    Ascomycota, including yeasts such as Candida (the most common fungal infection); filamentous fungi such as Aspergillus; Pneumocystis species; and dermatophytes, a group of organisms causing infection of skin and other superficial structures in humans.^[6]
    
  • 
    Basidiomycota, including the human-pathogenic genus Cryptococcus.^[7]
    
• 
  Parasites, which are usually divided into:^[8]
  
  • 
    Unicellular organisms (e.g. malaria, Toxoplasma, Babesia)
    
  • 
    Macroparasites^[9] (worms or helminths) including nematodes such as parasitic roundworms and pinworms, tapeworms (cestodes), and flukes (trematodes, such as schistosomes). Diseases caused by helminths are sometimes termed infestations, but are sometimes called infections.
    
• 
  Arthropods such as ticks, mites, fleas, and lice, can also cause human disease, which conceptually are similar to infections, but invasion of a human or animal body by these macroparasites is usually termed infestation.
  
• 
  Prions (although they do not secrete toxins)
  

Signs and symptoms[edit]
The signs and 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.^[10]
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.^[11]
Bacterial or viral[edit]
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.^[12] Distinguishing the two is important, since viral infections cannot be cured by antibiotics whereas bacterial infections can.^[13]

Comparison of viral and bacterial infection
Characteristic	Viral infection	Bacterial infection
Typical symptoms	In general, viral infections are systemic. This means they involve many different parts of the body or more than one body system at the same time; i.e. a runny nose, sinus congestion, cough, body aches etc. They can be local at times as in viral conjunctivitis or "pink eye" and herpes. Only a few viral infections are painful, like herpes. The pain of viral infections is often described as itchy or burning.^[12]	The classic symptoms of a bacterial infection are localized redness, heat, swelling and pain. One of the hallmarks of a bacterial infection is local pain, pain that is in a specific part of the body. For example, if a cut occurs and is infected with bacteria, pain occurs at the site of the infection. Bacterial throat pain is often characterized by more pain on one side of the throat. An ear infection is more likely to be diagnosed as bacterial if the pain occurs in only one ear.^[12] A cut that produces pus and milky-colored liquid is most likely infected.^[14]
Cause	Pathogenic viruses	Pathogenic bacteria

Pathophysiology[edit]

Chain of infection; the chain of events that lead to infection
There is a general chain of events that applies to infections, sometimes called the chain of infection.^[15] The chain of events involves several steps – which include the infectious agent, reservoir, entering a susceptible host, exit and transmission to new hosts. Each of the links must be present in a chronological order for an infection to develop. Understanding these steps helps health care workers target the infection and prevent it from occurring in the first place.^[16]