Infectious disease or communicable disease is a disease caused by a biological agent such as by a virus, bacterium or parasite. This is contrasted to physical causes, such as burns or chemical ones such as through intoxication.

Infectious diseases are the invasion of a host organism by a foreign replicator, generally microorganisms, often called microbes, that are invisible to the naked eye. Microbes that cause illness are also known as pathogens. The most common pathogens are various bacteria and viruses, though a number of other microorganisms, including some kinds of fungi and protozoa, also cause disease. Prions are borderline, and memes would not usually be considered in this scope. An infectious disease is termed contagious if it is easily transmitted from one person to another.

An organism that a microbe infects is known as the host for that microbe. In the human host, a microorganism causes disease by either disrupting a vital body process or stimulating the immune system to mount a defensive reaction. An immune response against a pathogen, which can include a high fever, inflammation, and other damaging symptoms, can be more devastating than the direct damage caused by the microbe.

For MT purposes it is necessary to know and remember the names of the diseases and also the pathogens causing them. A very detailed knowledge is not required. A once go through all the diseases would however be good in the long run.

Agents and Vectors

Infectious disease requires an agent and a mode of transmission (or vector). A good example is malaria, which is caused by Plasmodial parasites, chiefly Plasmodium falciparum but does not affect humans unless the vector, the Anopheles mosquito, is around to introduce the parasite into the human bloodstream.

The vector does not have to be biological. Many infectious diseases are transmitted by droplets which enter the airway (e.g. common cold and tuberculosis).

The top three single agent disease killers are HIV/AIDS, TB, and malaria. Lower respiratory infections (which include various pneumonias) and diarrheal diseases are caused by many different viruses, bacteria or parasites.

Childhood diseases include pertussis, poliomyelitis, diphtheria, measles, and tetanus. Children also make up a large percentage of lower respiratory and diarrheal deaths.

Tropical diseases include trypanosomiasis, Chagas disease. schistosomiasis, leishmaniasis, lymphatic filariasis, and onchocerciasis.

From 1993 to 2002, the death ranking for AIDS went up from 7th to 2nd and Hepatitis B went down from 6th to 11th. While the number of deaths has decreased in almost every disease, it has increased four-fold in AIDS.

Historic pandemics

A pandemic (or global epidemic) is a disease that affects people over an extensive geographical area.

The Influenza Pandemic of 1918 or Spanish Flu killed 25 to 40 million in 1 year (about 2% of world population of 1.7 billion). Influenza now kills about 250,000 to 500,000 worldwide each year. The Black Death of 1347 to 1352 killed 25 million in Europe over 5 years (estimate of 25 to 50% of Europe, Asia, and Africa - world population was 500 million).

Diagnosis and therapy

The field of infectious diseases also occupies itself with the diagnosis and therapy of infection.

Diagnosis

Diagnosis is initially by medical history and physical examination, and imaging (such as X-rays), but the principal tool in infectious disease is the microbiological culture. In a culture, a growth medium is provided for a particular agent. After inoculation of a specimen of diseased fluid or tissue onto the medium, it is determined whether bacterial growth occurs. This works for a number of bacteria, for example Staphylococcus or Streptococcus.

Certain agents cannot be cultured, for example the above-mentioned Treponema pallidum and most viruses. The first serological markers were developed to diagnose syphilis (the Wassermann test, later replaced by the VDRL and TPHA tests). Serology involves detecting the antibodies against an infectious agent in the patient's blood. In immunocompromised patients (e.g. AIDS), serology can be troublesome, because the antibody reaction is blunted.

A more recent development is direct detection of viral proteins and/or DNA in blood or secretions. This can be done by PCR (polymerase chain reaction), involving the amplification of viral DNA and its subsequent detection with anti-DNA probes.

List of infectious diseases

Viral infectious diseases

AIDS – AIDS Related Complex
Chickenpox (Varicella) 
Common cold 
Cytomegalovirus Infection 
Colorado tick fever 
Dengue fever 
Ebola haemorrhagic fever 
Epidemic parotitis 
Flu 
Hand, foot and mouth disease 
Hepatitis 
Herpes simplex 
Herpes zoster 
HPV 
Influenza 
Lassa fever 
Measles 
Marburg haemorrhagic fever 
Infectious mononucleosis
Mumps
Poliomyelitis
Progressive multifocal leukencephalopathy
Rabies
Rubella
SARS
Smallpox (Variola)
Viral encephalitis
Viral gastroenteritis
Viral meningitis
Viral pneumonia
West Nile disease
Yellow fever

Bacterial Infectious Diseases

Anthrax
Bacterial Meningitis
Brucellosis
Bubonic plague
Campylobacteriosis
Cat Scratch Disease
Cholera
Diphtheria
Epidemic Typhus
Gonorrhea
Impetigo
Hansen's Disease
Legionellosis
Leprosy
Leptospirosis
Listeriosis
Lyme Disease
Melioidosis
MRSA infection
Nocardiosis
Pertussis
Pneumococcal pneumonia
Psittacosis
Q fever
Rocky Mountain Spotted Fever or RMSF
Salmonellosis
Scarlet Fever
Shigellosis
Syphilis
Tetanus
Trachoma
Tuberculosis
Tularemia
Typhoid Fever
Typhus
Whooping Cough or Diptheria

Parasitic Infectious Diseases

African trypanosomiasis
Amebiasis
Ascariasis
Babesiosis
Chagas Disease
Clonorchiasis
Cryptosporidiosis
Cysticercosis
Diphyllobothriasis
Dracunculiasis
Echinococcosis
Enterobiasis
Fascioliasis
Fasciolopsiasis
Filariasis
Free-living amebic infection
Giardiasis
Gnathostomiasis
Hymenolepiasis
Isosporiasis
Kala-azar
Leishmaniasis
Malaria
Metagonimiasis
Myiasis
Onchocerciasis
Pediculosis
Pinworm Infection
Scabies
Schistosomiasis
Taeniasis
Toxocariasis
Toxoplasmosis
Trichinellosis
Trichinosis
Trichuriasis
Trypanosomiasis

Fungal infectious diseases

Aspergillosis
Blastomycosis
Candidiasis
Coccidioidomycosis
Cryptococcosis
Histoplasmosis
Tinea pedis

Prion Infectious Diseases

Bovine spongiform encephalopathy
Creutzfeldt-Jakob disease
Kuru

Infectious Diseases