GET AN INJECTION of gamma globulin before traveling to underdeveloped countries, nontourist areas of the tropics and even to rural areas of Southern Europe. The reason: to avoid infectious hepatitis. Today more Americans are returning from abroad with infectious hepatitis than with any other serious disease.

The existence of a relationship between travel and infectious hepatitis has been known for a long time. But until recently, public health officials were only able to guess at the incidence and apparently greatly underestimated it. The relationship between infectious hepatitis and travel is not as obvious nor as definite as between, say, travel and malaria or yellow fever.

Infectious hepatitis has a long incubation--usually many months--from the time of contact to the onset of symptoms. This makes it difficult to pinpoint the time and place of infection. Also, unlike malaria and yellow fever, the disease is often contracted without ever leaving home.

In fact, most cases are contracted locally, and many are subclinical. That means you can have the disease, and become immune, without knowing it. Thus in any group of travelers, some are already immune, others are not. In mild cases the symptoms resemble flu. In full-blown cases the symptoms are easy to recognize: jaundice--a yellow discoloration of the skin, severe loss of appetite, weight loss, fever, extreme tiredness.

Fortunately, infectious hepatitis is a preventable disease for the traveler. Here's what to know:

The chances of contracting infectious hepatitis vary dramatically in different parts of the world. In North America, Northern and Central Europe and Israel the chances are no worse if you travel than if you stay home. In Southern Europe, the risk is about 10 times greater than in the previous areas. The worst: Central Africa. And only slightly better: Central and South America. In these areas, the chance of picking up a case of infectious hepatitis is more than a hundred times greater than when traveling in North America.

Infectious hepatitis is caused by a virus that is most commonly found in sewage and enters the body via sewage-contaminated food and water--the same way traveler's diarrhea is acquired. However, infectious hepatitis virus is more difficult to destroy than the bacteria that cause most cases of traveler's diarrhea--it takes more boiling, more chemicals, more diligent hand-washing.

Within each geographical area, the risk will vary with how you travel and how much caution you exercise. Obviously, the better the hotels and the restaurants you patronize, the lower the chance of getting infectious hepatitis. In areas of great risk, avoid all uncooked and questionably cooked seafood, probably the most common single source of the virus.

Minimize the water you swallow when swimming in offshore waters. And avoid swimming pools if you are not sure of adequate chlorination.

Gamma globulin is not a vaccine; It is obtained from the blood of individuals who are immune to infectious hepatitis. When injected into a nonimmune person it gives immunity for one to three months, then disappears. Vaccines like those against yellow fever and cholera are weakened disease organisms. They harmlessly multiply in the body and thus give long-lasting immunity.

Repeat gamma globulin injections every three months if you remain in an area of high incidence.