contagious viral disease marked by fever, cough, conjunctivitis, and a characteristic rash. Measles is commonest in children (childhood disease and disorder) but may appear in older persons who have escaped it earlier in life. Infants are immune up to four or five months of age if the mother has had the disease. Immunity to measles following an attack is usually lifelong.

Measles is so highly communicable that the slightest contact with an active case may infect a susceptible person. After an incubation period of about 10 days, the patient develops fever, redness and watering of the eyes, profuse nasal discharge, and congestion of the mucous membranes of the nose and throat—symptoms often mistaken for those of a severe cold. This period of invasion lasts for 48 to 96 hours. The fever increases with appearance of a blotchy rash, and the temperature may rise as high as 40 °C (about 105 °F) when the rash reaches its maximum. Twenty-four to 36 hours before the rash develops, there appear in the mucous membranes of the mouth typical maculae, called Koplik spots—bluish-white specks surrounded by bright red areas about ¹/₃₂ inch (0.75 mm) in diameter. After a day or two the rash becomes a deeper red and gradually fades, the temperature drops rapidly, and the catarrhal symptoms disappear.

No drug is effective against measles; the only treatment required is control of fever, rest in bed, protection of the eyes, care of the bowels, and sometimes steam inhalations to relieve irritation of the bronchial tree. When no complications occur, the illness lasts 10 days. Uncomplicated measles is seldom fatal; deaths attributed to measles usually result from secondary bronchopneumonia caused by bacterial organisms entering the inflamed bronchial tree. On the other hand, complications of measles are frequent and include a superimposed bacterial ear infection or pneumonia or a primary measles lung infection. encephalitis is a rare occurrence. Measles virus can invade various organ systems and cause hepatitis, appendicitis, and gangrene of the extremities. A large percentage of cases of severe measles are associated with inadequate intake of vitamin A, and there is evidence that treatment with vitamin A may reduce measles complications.

Mortality caused by measles declined steadily in the 20th century as the health of children and infants improved and effective treatment of complications became possible through the use of sulfonamide and antibiotic drugs. The widespread use of measles vaccine, beginning in the late 1960s, raised hopes for the eventual eradication of the disease; but, contrary to expectations, the incidence of measles remains high worldwide. The main problem is that the vaccine is not given to infants before the age of nine months, when the disease is most serious in the less-developed countries. Another problem is that the measles vaccine is a live vaccine, and it rapidly becomes inert if exposed to warm temperatures; 10 minutes in sunlight is sufficient to kill it. This sensitivity is a great hindrance to its use in tropical areas. Research is currently directed toward development of a more stable vaccine. In developed countries, a measles vaccine is commonly given at 12 to 15 months of age as part of a combined measles-mumps-rubella (MMR) vaccine.

Measles must be differentiated from other disorders accompanied by an eruption. In roseola infantum, a disease seen in babies, a measleslike rash appears after the child has had a high temperature for two or three days, but there is no fever at the time of the rash. German measles ( rubella) can be superficially differentiated from measles by the shorter course of the disease and mildness of the symptoms. Sometimes the rashes of scarlet fever, serum reactions, and other conditions may, on certain parts of the body, look like measles. Drugs that may produce rashes similar to measles are phenobarbital, diphenylhydantoin, the sulfonamides, phenolphthalein, and penicillin.

On very rare occasions, persistent infection with a mutant measles virus can cause a degenerative central nervous system disease called subacute sclerosing panencephalitis (SSPE), in which there is a gradual onset of progressive behavioral and intellectual deterioration. Motor incoordination and impairment of speech and sight subsequently develop. The final stages of stupor, dementia, blindness, and death occur within six to nine months. There is no treatment for SSPE.