MALARIA IS A risk no traveler to the tropics can afford to take lightly. Last year there were 680 cases of malaria in the United States, and roughly half of them occurred in Americans who had recently returned from abroad.

The disease is also a rapidly growing problem in this country. Although no region of the United States is tropical enough to harbor malaria, the number of imported cases has increased by 40 percent in the past two years. Disease experts attribute the increase to the influx of refugees and the rising popularity of travel to tropical countries.

Malaria can be dangerous -- even fatal -- for Americans, because they usually get it much more severely than those who have lived in a malarious area long enough to have built up immunity through repeated exposure. The six people who died of malaria in the United States in 1978 were all American citizens.

The approximately 300 cases of malaria that occur annually in returning travelers are a frustration for public health officials for two reasons. One is that malaria is preventable: Travelers who take a weekly dose of the drug chloroquine during, and for six weeks after, a trip to the tropics are usually protected.

The second is that malaria is curable -- yet, faced with a disease that is seldom seen in the United States, American doctors sometimes make the diagnosis too late to save the patient. Tropical disease experts emphasize the importance of patients' informing their doctors that they have been abroad -- even if the trip ended two or three years before an illness begins.

There are actually several different kinds of malaria, but all of them are caused by a parasite that enters the bloodstream when the victim is bitten by an infected anopheles, mosquito. Worldwide, the disease is a public health problem of huge proportions. According to a World Health Organization estimate, 1 million children die of malaria each year in Africa alone, and there are probably several hundred million cases annually in tropical countries.

Once the malaria parasite enters the victim's blood, it quickly travels to the liver and reproduces there over the next week or 10 days. Throughout this period, there are no symptoms. It is only when the parasites move from the liver and begin to infect red blood cells that the classic symptoms of malaria appear -- a high fever, chills, headache and muscle aches.

The parasites live and reproduce within the blood cells, and each bout of fever coincides with the breaking open of infected blood cells to release a swarm of new parasites. Since malaria organisms have a two- or three-day reproductive cycle, the fever classically comes and goes every two or three days -- but the disease is not this regular in every patient.

The high fever produces dehydration, and this combined with anemia from loss of blood cells can lead to heart and kindney failure, coma and death. One form of the disease, falciparum malaris, is known as "malignant malaria." According to Cr. G.m. jEffery, a malaria expert at the federal Center for Disease Control in Atlanta, the outlook is grim for an American who contracts falciparum malaria unless treatment is swift.

As a traveler, the best way to protect yourself is to research the malaria risk in the countries you intend to visit, get a prescription for the right anti-malarial drug, and taske it religiously. In most cases, the drug will be chloroquine, but in some areas of the world -- parts of Asia and South America, Panama and New Guinea -- the falciparum parasite has become resistant to chloroquine. The disease control center recommends that travelers to these areas begin by taking chloroquine, but obtain another drug (Fansidar, or pyrimethamine-sulfadoxine) from health officials once they reach the malarious area. Fansidar is not available in the United States.

Even if you are taking chloroquine, it is a good idea to minimize your exposure to malaria by wearing long-sleeved clothing, using mosquito repellent, sleeping behind screens or netting, and staying indoors in the evening, when Anopheles mosquitos bite.

Most people think of malaria if they are planning a safari or a trans-Asiatic trek, but the risk exists even in countries loaded with tourist amenities. For instance, there is amalaria in much of Mexico, most of Central America, Haiti and the Dominican Republic, Egypt, Algeria, Jordan and even Turkey.

Even if you take a preventive medicine, you cannot forget about malaria when you take your last chloroquine tablet. Two kinds of malaria can maintain in the liver a reservoir of parasites that are unaffected by chloroquine, even though the drug kills malaria parasites in blood cells. So any time from a few months to several years after he stops taking the drug, a traveler can come down with the disease, when his trip is already a distant memory.

These are the cases that are apt to stymie American doctors. "They need to ask the question, 'Where have you been?'" said Dr. Jeffery.

Travelers can obtain information on malaria and other health risks by ordering the Center for Disease Control's booklet "Health Information for International Travel" (number 017-001-00399-9) from the Superintendent of Documents, U.S. Government Printing Office, Washington D.C. 20402. The price is 50 cents a copy, postpaid.