Cholera is caused by a bacterial infection of the intestine. In many cases the infection is mild — sometimes producing no symptoms at all. But approximately one in 20 people infected with cholera has a serious case, with symptoms including severe diarrhea, vomiting, and leg cramps. These symptoms quickly cause dehydration and shock, and can result in death within hours if the infected person doesn’t receive treatment.

Cholera is typically transmitted by contaminated food or water. In areas with poor treatment of sewage and drinking water, the feces of people with cholera can enter the water supply and spread quickly, resulting in an epidemic. The cholera bacterium may also live in the environment in some coastal waters, so shellfish eaten raw can be a source of cholera in affected areas.

In the U.S., as in most other industrialized nations, cholera was common in the 1800s but has been virtually wiped out by modern sewage and water treatment systems. Travelers to areas where cholera is endemic may be exposed to the bacterium and can bring it back when they return. The risk of this is very low, however, and can be avoided by taking simple precautions when eating and drinking in areas with epidemic cholera. Also, because the disease isn’t likely to spread through casual contact, returning travelers typically don’t cause widespread cholera outbreaks.

Although some cholera infections can be very severe, and even fatal, the disease can be easily and successfully treated by quickly replacing the fluid and salts lost through diarrhea. With adequate treatment, fewer than 1 percent of cholera patients die from the disease.

Diphtheria is a serious bacterial disease that affects the respiratory system or the skin. Respiratory diphtheria causes a sore throat and fever, and sometimes swelling of the neck. In severe cases it can cause a membrane to form over the throat, which results in breathing problems. Cutaneousdiphtheria affects the skin, causing infected lesions to form. Diphtheria can lead to coma and death if it goes untreated.

An infected person usually spreads the disease by coughing or sneezing. The person expels droplets containing the diphtheria bacteria, which are then inhaled by another person. The disease is treated by hospitalization and antibiotics.

Diphtheria was once very common in the U.S., with hundreds of thousands of cases occurring every year. Since the introduction of a vaccine in the 1920s, cases of diphtheria in the U.S. have declined greatly, with less than one case reported each year since 2000. But while mandatory vaccines for schoolchildren have gone a long way toward controlling diphtheria in the U.S., the disease is still endemic in many developing countries.

Malaria is caused by a parasite transmitted by infected mosquitoes. Symptoms occur ten days to four weeks after being bitten and are similar to symptoms brought on by the flu — chills, fever, sweating, headache, and muscle pain. In serious cases, malaria may cause vomiting, anemia (iron deficiency), kidney failure, coma, and death.

The disease can be prevented by taking anti-malarial drugs and by avoiding mosquito bites in areas where malaria infection is common. Those infected with malaria can be treated with prescription drugs, which are most effective when taken early in the course of the disease.

The parasite that most often causes malaria needs warm temperatures to grow and thrive, so the disease is typically found in tropical and subtropical countries. Malaria was once common in most of Europe and North America, but effective mosquito control and other measures have nearly eradicated the disease in these regions. While only about 1300 cases of malaria are reported in the U.S. each year, 300 to 500 million cases occur around the world — mostly in developing countries — resulting in more than 1 million deaths from malaria globally each year.

Measles is a highly contagious disease caused by a virus. Early symptoms include fever, cough, red eyes, and a runny nose. During the first few days, the characteristic measles rash appears, beginning with white spots in the mouth and spreading to a red rash that covers the entire body. The rash typically lasts four to seven days. Severe cases of measles can cause diarrhea, ear infection, pneumonia, encephalitis (swelling of the brain), and death.

The measles virus is easily spread through airborne droplets expelled by coughing or sneezing, and can live in the air for up to two hours after an infected person has been present. After exposure, the virus lives in the body for about two weeks before symptoms appear. There is no specific remedy available for measles, so treatment usually consists of bed rest and easing symptoms.

Measles are still quite common, with more than 20 million people infected around the world each year. In the U.S., a widespread immunization campaign has successfully controlled the disease, and fewer than 150 cases have been reported since 1997. About half of these cases result from visits to other countries where measles is still endemic — including some developed countries in Europe and Asia.

Mumps is a contagious viral disease that causes painful swelling of the salivary glands. As a result, people infected with mumps sometimes appear to have “chipmunk cheeks.” Other symptoms include fever, headache, sore muscles, and fatigue. Serious complications are rare, and may include encephalitis (swelling of the brain), inflammation of the sex organs, and deafness.

The mumps virus is transmitted by contact with the respiratory secretions of an infected person. Like measles, mumps has a relatively long incubation period, with symptoms appearing more than two weeks after exposure. There are no specific treatments available for mumps, but the disease can be prevented by immunization. Following the introduction of the mumps vaccine in 1967, reported mumps cases had declined to fewer than 1,000 per year in the U.S. In recent years, however, mumps cases have increased.

Influenza, more commonly known as “the flu,” is caused by a contagious virus. Symptoms include body aches, sore throat, headache, fever, coughing, and chills. Perhaps because influenza is so common, misconceptions about the disease abound. Often, people who experience a bad cold say they have the flu, but this is incorrect: Unlike influenza, colds rarely cause headaches or fever. And despite widespread use of the term “stomach flu,” true influenza does not cause gastrointestinal symptoms.

The flu is spread through airborne respiratory secretions. Symptoms can be serious, and the disease can be fatal — especially for babies, the elderly, and people with weakened immune systems. When influenza pandemics occur, they spread quickly, often killing large numbers of previously healthy people. From 1918 to 1919, a flu pandemic hit the U.S. in three waves. By the pandemic’s end, it had spread over the entire globe and killed about 20 million people. Influenza pandemics occurred again in 1957–1958, and in 1968–1969, although they were nowhere near as severe as the pandemic of 1918–1919.

Influenza is still very common in the U.S.: According to the Centers for Disease Control and Prevention, between 5 percent and 20 percent of Americans get the flu every year, and about 36,000 cases are fatal. The disease can be treated by antiviral medicines, and a seasonal vaccine is available to protect against it. Because the virus changes every year, the vaccine must be reformulated yearly. Doctors recommend that high-risk members of the population get a vaccine at the beginning of each flu season.