Infectious Diseases

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