DHAKA, Bangladesh THE man staggered as he walked in and would have collapsed on the floor had a friend not been holding him up. Soon he was stretched out on a cot in a long, spotless, wardlike room that held perhaps 125 similarly stricken people, more than half of them children who appeared to be less than 5 years old.
Pale and sunken-eyed, some of the children whimpered or cried as their mothers, wrapped in gaily colored Bengali saris, ministered to them, but most were silent.
No cyclone or war was responsible. All were victims of a medical disaster of gigantic proportions and lethal powers. It is not the plague, malaria or yellow fever. It is simple, miserable, inelegant diarrhea, the most widespread and pervasive health problem in underdeveloped nations, and the biggest killer of children on the earth.
''Delhi belly'' is a joke among world travelers, although not always to those who have had it. But in Asia, Africa and Latin America, it is an inescapable fact of existence, and for those under 5 a very real threat to life.
Four million to five million children die each year from the dehydration caused by severe diarrhea. Various studies indicate there are one billion to two billion cases of the malady annually. Virtually every child in the third world will have the disease more than once in his or her lifetime. With some common diarrheal diseases, a child or even an adult can die in a day, so severe and rapid is the dehydration.
Yet, experts in the field of diarrheal diseases say that knowledge about such diseases has been limited, and that prevention and treatment programs have been haphazard at best and nonexistent at worst. Now advances in both understanding and controlling the diseases are being made, many of them here in Dhaka at the world's only internationally sponsored center for the study and treatment of diarrhea.
''Ten years ago, we knew about 20 percent of the causes of diarrheal disease,'' said Dr. K.M. Sultanul Aziz, associate director of the International Center for Diarrheal Disease Research, Bangladesh. ''But, we have proceeded rapidly. Today we know the cause of 80 to 90 percent.''
Until 1978, the center was a cholera laboratory operated by the Bangladesh Government. Now, supported and governed by 19 countries and agencies and staffed by professionals from 14 nations, it has become recognized as the world's primary institution for research, collection of information and development of treatment methods for diarrheal diseases. Its director, Dr. W.B. Greenough 3d, is an American.
Among other things, investigators and physicians at the center have discovered that cholera and amoebic dysentery, perhaps the two best-known and traditionally most-feared forms of diarrheal disease, are not the most lethal among this group of ailments. They have also found, contrary to conventional wisdom, that diarrheal patients absorb food almost normally and should therefore eat normally. They are trying to clarify the extent to which the body develops immunity to some of the diseases.
And perhaps most important, they are propagating a simplified and effective oral method of treatment that can be used by families in the poorest and most illiterate villages, using easily obtainable materials and ingredients.
What might be called the Dhaka doctrine holds that medication is unnecessary in most cases of severe diarrhea. The best thing to do, say the Dhaka doctors, is to let the body purge itself naturally, while at the same time replacing the fluids, nutrients and minerals lost through dehydration. They have devised a simple method for doing this orally, using a mixture of water, salt, sodium bicarbonate, potassium chloride and sugar.
Where some of the ingredients are unavailable, as is likely to be the case in rural areas, they have found that an even simpler solution of a three-finger pinch of salt and a fistful of sugar to a half liter of water works very well. And where sugar is not available, researchers here have found pulverized rice or wheat, the staples of most third world diets, will suffice.
Some 400 to 500 patients a day are treated at the center's new hospital here, and more drink the cure at an older center at the town of Matlab, south of here.
The effects of diarrheal disease involves the body's attempt to flush from the digestive tract harmful bacteria, protozoa and viruses that invade it through contaminated water or food. Death is caused not by the infection itself, but by the body being rapidly drained of its vital fluids. The most likely victims are children weakened by malnutrition, as so many are in underdeveloped countries.
Since contaminated water is the norm in the third world, virtually the entire population of Asia, Africa and Latin America is exposed to disease-carrying bacteria. Small children, coming from the protected environment of the uterus, are wide open to infection because they have not built up the immunity some diarrheal infections provide. The same is true of adult travelers from Western countries, where pure drinking water is the rule.
Studies in Bangladesh have shown that the two most prevalent and serious diarrheal diseases are caused by the bacterium Escherichia coli, which makes its way into water supplies through contamination by fecal matter, and rotavirus, which can be transmitted not only through food or water, but also by skin contact from person to person in a handshake. One study showed that in children under 2, 73 percent of all cases of diarrhea were caused by one of these two pathogens.
Both are regarded as very dangerous: rotavirus because it is the predominant cause of death in children under 3, and E. coli because it can cause very rapid and catastrophic dehydration.
The incidence of infection by rotavirus drops in the later years of life because, scientists believe, it confers a high level of immunity on children who survive the first attack.
Four other bacteria, ''including our old friend cholera,'' says Dr. Aziz, also cause disabling or fatal diarrhea, but affect fewer children. When cholera does strike, however, death can come within 24 hours if treatment is not prompt. Dr. Aziz added that the cholera vaccine required of travelers to some countries has now been found to be ineffective.
The other bacteria involved are shigella, which can also be transmitted hand-to-hand and then to food; salmonella, transmitted in food; and campylobacter, which can be carried by poultry and domestic animals.
Among the protozoa, or one-celled animals, the two most important causes of diarrhea are amoebas and a parasite called Giardia lamblia. Dr. Aziz said he considered the purification of third world water supplies a hopeless task for the foreseeable future. The next line of defense lies in certain preventive measures.
The experts recommend that people in underdeveloped countries wash their hands thoroughly with soap and hot water after defecation and before eating. Drink nothing except boiled water, they say, or water from a well-proved pure source, or liquids from a sealed bottle you have opened, or seen opened, yourself.
Eat only thoroughly cooked food that is still hot (even the food in street bazaars is all right if this rule is followed). Eat no fresh fruit except what you peel yourself. Eat no raw vegetables except those that have been properly treated in a special germ-killing wash, such as a solution of potassium permanganate. Make sure that dishes, glasses, cups and utensils have been washed in boiling water.
Visiting westerners and middle-class local people commonly try to observe such precautions. But in most third world countries, said Dr. Aziz, it is ''unrealistic'' to expect their widespread adoption.
The Dhaka doctrine of oral rehydration holds that except in a few cases where a bacterial infection may move outside the digestive tract and invade other organs, medication should not be given because it impedes the development of immunity. And as in all viral diseases, the rotavirus is unaffected by antibiotics.
The protozoal infections are an exception to the nonmedication rule. It is considered important to treat them antibiotically to kill the live parasites.
Illustrations: photo of a patient in Banglasdesh being treated for diarrhea