At the end of the 19th cent., yellow fever was highly prevalent in the Caribbean, and a way of controlling it had to be found before construction of the Panama Canal could be undertaken. In 1900 an American commission headed by Walter Reed and including James Carroll, Jesse Lazear, and Aristides Agramonte gathered in the U.S. Army's Camp Columbia in Cuba. Through their experiments they proved the theory of C. J. Finlay that yellow fever was a mosquito-borne infection. Within the next few years, W. C. Gorgas, an army physician and sanitation expert, succeeded in controlling the disease in the Panama Canal Zone and other areas in that part of the world by mosquito-eradication measures. The later development of an immunizing vaccine and strict quarantine measures against ships, planes, and passengers coming from known or suspected yellow-fever areas further aided control of the disease.

Yellow fever begins suddenly after an incubation period of three to five days. In mild cases only fever and headache may be present. The severe form of the disease commences with fever, chills, bleeding into the skin, rapid heartbeat, headache, back pains, and extreme prostration. Nausea, vomiting, and constipation are common. Jaundice usually appears on the second or third day. After the third day the symptoms recede, only to return with increased severity in the final stage, during which there is a marked tendency to hemorrhage internally; the characteristic "coffee ground" vomitus contains blood. The patient then lapses into delirium and coma, often followed by death. During epidemics the fatality rate was often as high as 85%. Although the disease still occurs, it is usually confined to sporadic outbreaks.

Treatment of rheumatic fever is with penicillin, salicylates, and steroids; extended rest is usually necessary. Rheumatic fever may be prevented by prompt treatment of all streptococcal infections. Cardiac damage may possibly be avoided if prophylactic measures are taken after a first attack of rheumatic fever, i.e., long-term maintenance doses of antibiotics, to discourage streptococcal infections and recurrences of rheumatic fever. Rheumatic fever has declined in incidence in the industrialized countries, but has increased in prevalence in the Third World. See also streptococcus.

Ebola and Marburg are closely related, newly emergent viruses that have in recent years caused epidemics in central Africa, with very high rates of mortality. Hantavirus occurs in many different parts of the world and is spread to humans from field rodents via microscopic bits of their excretions that get into the air and are inhaled. It was originally known as a disease of Asia and Europe that primarily attacked the kidneys, but a more deadly pulmonary form of hantavirus infection has more recently caused numerous fatalities in the United States, Chile, and other countries. Lassa fever, also spread to humans from rodent excretions, occurs primarily in W Africa. Closely related to the Lassa virus are the Junin and Machupo viruses, which have caused outbreaks of hemorrhagic fever in South America. Yellow fever, transmitted by the bite of a mosquito, still occurs in tropical areas despite largely successful control efforts. Dengue hemorrhagic fever, also spread by mosquitoes, has in recent years caused many fatalities among children in tropical countries.

There is usually no specific treatment to combat the viruses that cause hemorrhagic fevers. An exception is the drug ribavirin, which has been effective in treating Lassa fever. Treatment generally consists of such supportive measures as the replacement of lost blood, the maintainence of fluid balance, and the alleviation of symptoms. Survival depends largely upon the virulence of the virus strain and the quality of treatment.

Dengue hemorrhagic fever, a severe form of the disease, can cause hemorrhage, shock, and encephalitis. It occurs when a person who has acquired immunity to one of the viruses that cause dengue fever is infected by a different dengue virus. It is a leading cause of death among children in Southeast Asia and in recent years has become increasingly prevalent in tropical America. There is no specific treatment for dengue fever except good nursing care. Both diseases can be controlled by eradicating the mosquitoes and destroying their breeding places.

The incubation period of Lassa fever is 3 to 17 days. Following fever and general malaise, later stages of the disease may include abdominal pain, diarrhea, vomiting, and petechiae, tiny purplish spots in the skin caused by leakage of blood from the capillaries. Heart and kidney failure may also occur in severe cases, and mortality is high, ranging from about 15% to, among pregnant women, as much as 60%. Treatment by injection of the antiviral drug ribavirin is often successful if begun early.

See also hemorrhagic fever.

Acute infectious tropical disease, sometimes occurring in temperate zones. Abrupt onset of headache, backache, fever, nausea, and vomiting is followed by either recovery with immunity or by higher fever, slow pulse, and vomiting of blood. Patients may die in a week. Jaundice is common (hence the name). One of the world's great plagues for 300 years, it is caused by a virus transmitted by several species of mosquitoes. Carlos Finlay suggested and Walter Reed proved this means of spread, leading to near elimination of the disease through mosquito control (see William Gorgas). Treatment consists of supportive care, particularly fever reduction. Control of mosquitoes near cities and live-virus vaccines—developed by Max Theiler (1899–1972), who won a 1951 Nobel Prize for his work—have made yellow fever completely preventable.

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Infectious disease of humans and domestic animals. It is characterized by gradual onset of fever, chills, sweats, weakness, and aches, and it usually ends within six months. It is named after the British physician David Bruce (b. 1855—d. 1931), who first identified (1887) the causative bacteria. Three main species in the genus Brucella commonly cause the disease in humans, who contract it from infected animals (goats, sheep, pigs, cattle). Brucellosis is rarely transmitted between humans but spreads rapidly in animals, causing severe economic losses. Drug therapy is not practical for animal brucellosis, but vaccination of young animals is useful. Infected animals must be removed from herds. Antibiotics are effective against acute disease in humans, in whom it can cause liver and heart problems if untreated.

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Acute infectious disease resembling typhus (and distinguished from it only in the 19th century). Salmonella typhi, usually ingested in food or water, multiplies in the intestinal wall and then enters the bloodstream, causing septicemia. Symptoms begin with headache, aching, and restlessness. High fever gradually develops, with delirium. A rash appears on the trunk. The sites where the bacilli multiplied become inflamed and may ulcerate, leading to intestinal bleeding or peritonitis. Patients become exhausted and emaciated; up to 25percnt die if not treated. Antibiotic treatment is effective. Patients can carry typhoid for weeks to months or years. Carriers can contaminate the food they handle. Prevention depends mainly on water and sewage treatment and excluding carriers from food-handling jobs.

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Often fatal viral disease of swine in Europe, North America, and Africa, transmitted by vehicles used to carry pigs, people dealing with them, and uncooked garbage in feed. Fever progresses to symptoms that include appetite loss; affected eyes and digestive tract; respiratory difficulty; rash; and inflamed mouth and throat. The pig moves reluctantly and staggers; later it cannot rise; coma follows. Antiserum is rarely effective. Survivors become chronically ill and can spread the virus. Illness must be reported, infected animals slaughtered, and quarantine instituted. A vaccine can control it. The African strain causes death sooner and has no effective prevention or treatment.

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Infectious disease of warm-blooded animals, caused by Bacillus anthracis, a bacterium that, in spore form, can retain its virulence in contaminated soil or other material for many years. A disease chiefly of herbivores, the infection may be acquired by persons handling the wool, hair, hides, bones, or carcasses of affected animals. Infection may lead to death from respiratory or cardiac complications (within 1–2 days if acute), or the animal may recover. In humans, anthrax occurs as a cutaneous, pulmonary, or intestinal infection. The most common type, which occurs as an infection of the skin, may lead to fatal septicemia (blood poisoning). The pulmonary form of the disease is usually fatal. Sanitary working environments for susceptible workers are critical to preventing anthrax; early diagnosis and treatment are also of great importance. In recent decades, various countries have attempted to develop anthrax as a weapon of biological warfare; many factors, including its extreme potency (vastly greater than any chemical-warfare agent), make it the preferred biological-warfare agent. Concerns about anthrax mounted in 2001 after it was found in letters mailed to members of the U.S. government and news agencies.

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Acute infectious disease caused by some types of streptococcus bacteria. Fever, sore throat, headache, and, in children, vomiting are followed in two to three days by a rash. The skin peels in about one-third of cases. After a coating disappears, the tongue is swollen, red, and bumpy (strawberry tongue). Glands are usually swollen. Complications frequently involve the sinuses, ears (sometimes with mastoiditis), and neck. Abscesses are common. Nephritis, arthritis, or rheumatic fever may occur later. Treatment involves penicillin, bed rest, and adequate fluid intake. Scarlet fever has become uncommon and much milder since the mid-20th century, independent of the use of antibiotics.

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Generalized disease caused by certain types of streptococcus bacteria. It occurs mostly in children and young adults. Symptoms may be mild or severe. Sudden fever, joint pain, and inflammation may begin days to weeks after a streptococcal infection, usually of the throat (see pharyngitis). Other symptoms may include skin nodules and rashes, chorea, abdominal pain, nosebleeds, and weight loss. Heart inflammation, with accompanying rapid heartbeat, murmurs, and enlargement, can lead to valve scarring, markedly shortening life. After recovery, survivors are prone to future attacks. Penicillin given when the initial infection is diagnosed can prevent it. Otherwise, salicylic acid derivatives or corticosteroids help the symptoms.

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Infectious disease with recurring fever, caused by several spirochetes of the genus Borrelia, transmitted by lice, ticks, and bedbugs. Onset is sudden, with high fever, which breaks within a week with profuse sweating. Symptoms return about a week later. There may be 2 to 10 relapses, usually decreasing in severity. Mortality usually ranges from 0 to 6percnt, up to 30percnt in rare epidemics. Central nervous system involvement causes various (usually mild) neurological symptoms. The first microscopic organisms clearly associated with serious human disease (1867–68), the spirochetes mutate repeatedly, changing their antigens so that the host's immunity no longer is effective, which produces the relapses. Antibiotics can be effective, but inadequate therapy may leave spirochetes alive in the brain, and they may reinvade the bloodstream.

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Infection of the female reproductive system after childbirth or abortion, with fever over 100 °F (38 °C) in the first 10 days. The inner surface of the uterus is most often infected, but lacerations of any part of the genital tract can give bacteria (often Streptococcus pyogenes) access to the bloodstream and lymphatic system to cause septicemia, cellulitis (cellular inflammation), and pelvic or generalized peritonitis. Severity varies. Puerperal fever has become very rare in developed countries but is still seen after abortions performed in unhygienic surroundings.

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Seasonal sneezing, nasal congestion, and tearing and itching of the eyes caused by allergy to the pollen of certain plants. These plants are chiefly those pollinated by the wind (e.g., ragweed in North America, timothy grass in Britain). Antihistamines and corticosteroids may provide temporary relief, but the most effective long-range treatment is desensitization. Unless properly treated, about one-third of patients with hay fever develop asthma.

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Common infection, caused by Epstein-Barr virus. It occurs most often at ages 10–35. Infected young children usually have little or no illness but become immune. Popularly called “the kissing disease,” it is spread mostly by oral contact with exchange of saliva. It usually lasts 7–14 days. The most common symptoms are malaise, sore throat, fever, and lymph-node enlargement. Liver involvement is usual but rarely severe. The spleen often enlarges and in rare cases ruptures fatally. Less frequent features include rash, pneumonia, encephalitis (sometimes fatal), meningitis, and peripheral neuritis. Relapse and second attacks are rare. Diagnosis may require blood analysis. There is no specific therapy.

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Abnormally high body temperature or a disease characterized by it. It most often occurs with infection. Normal core body temperature, measured orally, does not exceed 99°F (37.2°C). Up to 105°F (40.6°C), fever causes weakness and is best treated with aspirin, acetaminophen, or other antipyretic drugs. At 108°F (42.2°C) or more, it can lead to convulsions and death. In treatment, it is important to know the underlying cause. Fever appears to be a defense against infectious disease, stimulating leukocytes and increasing antibody production and perhaps killing or inhibiting bacteria and viruses that live within a narrow temperature range.

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Infectious, disabling mosquito-borne fever. Other symptoms include extreme joint pain and stiffness, intense pain behind the eyes, a return of fever after brief pause, and a characteristic rash. Dengue is caused by a virus carried by mosquitoes of the genus Aedes, usually A. aegypti, which also carries yellow fever. There are four strains of virus; infection with one type does not confer immunity to the remaining three. Treatment focuses on relieving symptoms. Patients should be isolated during the first three days, when mosquitoes can pick up the disease from them. Prevention relies on mosquito control.

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Typhus-like disease first seen in the Rocky Mountain region, caused by the bacterium Rickettsia rickettsii (see rickettsia) and transmitted by various ticks. In severe cases the rash bleeds more and is especially prominent on the wrists and ankles. Central nervous system involvement causes restlessness, insomnia, and delirium. Prostration may progress to coma, with death possible in a week or more. Mortality increases with age. Recovery is slow but usually complete as visual disturbances, deafness, and mental confusion pass. Prompt antibiotic treatment hastens it and reduces mortality. Prevention depends on avoiding tick bites, by wearing long, light-coloured clothing and insect repellent and inspecting for ticks. A vaccine reduces the risk of infection somewhat and of death greatly.

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