Pharyngitis causes

Causes
The pharynx is often the first site of infection for many contagious diseases such as the common cold. This is because viruses and bacteria often settle in this part of the body after a person inhales dust or water vapor containing the microorganism. Infection can also arise when a person touches their nose or mouth after having touched an object shared with another person with the disease. The foreign invader reproduces rapidly after settling on the body tissue.

Viral sore throats
These comprise about 90% of all infectious cases and can be a feature of many different types of viral infections.
 * Adenovirus - the most common of the viral causes. Typically the degree of neck lymph node enlargement is modest and the throat often does not appear red, although is very painful.
 * Orthomyxoviridae which cause influenza - present with rapid onset high temperature, headache and generalised ache. A sore throat may be associated.
 * Infectious mononucleosis ("glandular fever") caused by the Epstein-Barr virus. This may cause significant lymph gland swelling and anexudative tonsillitis with marked redness and swelling of the throat. The heterophile test can be used if this is suspected.
 * Herpes simplex virus can cause multiple mouth ulcers.
 * Measles
 * Common cold

Group A streptococcal

 * See also Strep throat

The most common bacterial agent is streptococcus. Unlike adenovirus, there tends to be greater generalized symptoms and more signs to find. Typically enlarged and tender lymph glands, with bright red inflamed and swollen throat, the patient may have a high temperature, headache, and aching muscles (myalgia) and joints (arthralgia). It may be impossible to distinguish between viral and bacterial causes of sore throat.

Some immune-system meditated complications may occur:
 * Scarlet fever with its vivid rash, although the milder disease seen after the 1950's suggests that the bacteria may have mutated to less virulent illness and some doctors now call this scarlatina (literally a 'little scarlet fever')
 * Historically the most important complication was of the generalised inflammatory disorder of rheumatic fever which could later result in Rheumatic heart disease affecting the valves of the heart. Antibiotics may reduce the incidence of this complication to under one-third. However the incidence of rheumatic fever in developed-regions of the world remains low even though the use of antibiotics has been declining. This may be a result of a change in the prevalence of various strains of bacteria.  In underdeveloped regions, untreated streptococcal infection can still give rise to rheumatic heart disease and may be due to environmental factors, or reflect a genetic predisposition of the patient to the disease.
 * Post-streptococcal glomerulonephritis is an inflammation of the kidney. It is disputed whether antibiotics might reduce the small risk of this or not.
 * Very rarely there may occur a secondary infection behind the tonsils which may cause a life-threatening septicaemia (Lemierre's syndrome)

Diphtheria
Diphtheria is a potentially life threatening upper respiratory infection caused by Corynebacterium diphtheriae which has been largely eradicated in developed nations since the introduction of childhood vaccination programs, but is still reported in the Third World and increasingly in some areas in Eastern Europe. Antibiotics are effective in the early stages, but recovery is generally slow.