The meal was simple: canned tuna and sliced onions. But it killed Marita Saldano`s six-week-old son.

The baby died of dehydration caused by vomiting and severe diarrhea-another victim of a frightening cholera epidemic that has sickened 177,103 Peruvians and killed 1,300 others since late January.

Saldano, who lives in a straw shack in Lima`s Olivos del Pro slum, doesn`t know how her baby contracted cholera, which also sickened two of her other children and her husband. But Lilsio Infantas, a local health worker, said the onions they ate were contaminated.

''The vegetables people eat in this area are infected. They are grown on land irrigated by the Chillon River, which has raw sewage dumped into it. The people also drink the river water untreated,'' he said, walking through the area`s maze of makeshift huts that are dissected by streams of raw sewage.

''I ate a little seviche (a popular dish of raw fish marinated in lime juice), and a few hours later I was violently ill. I had stomach cramps, a pounding in my head and pain all over,'' said Yolanda Condori, 17, who was recovering last week in the only hospital in Comas, a slum of 1 million people 15 miles north of Lima.

Cholera is a disease of the poor, spread by contaminated water and infected people, that experts say is likely to become a recurring health problem in the 1990s, both in Peru and throughout Latin America.

''The situation is terrible. I don`t think we can stop it,'' said Enrique Fernandez, an adviser on cholera to Peru`s minister of health. ''The same squalor that allowed cholera to spread here exists in most other Latin America countries. Cholera is the symptom. The real disease is poverty.''

The cholera epidemic, South America`s first since 1895, appeared three months ago in Peru`s northern ports and quickly spread south to the slums, called pueblo jovenes, that ring this capital of 7 million people.

Since then, the disease has moved east to the highlands and to the Amazon jungle basin. It has also struck neighboring countries-Ecuador, Colombia, Chile and Brazil. Of those countries, the situation is most severe in Ecuador, where more than 6,000 cases and 170 deaths have been reported.

Experts in Peru speculate the cholera bacterium was brought to the country by ship from Asia. To catch the disease, a person must swallow the bacterium, though 80 percent of those infected don`t get ill, in part because stomach acids neutralize the bacterium.

But anyone who ingests the bacterium becomes a carrier, spreading it through excrement that usually flows untreated into Peru`s rivers and coastal waters. Most Peruvians get the disease by drinking contaminated water, or by eating contaminated fish and other food. The main symptoms are high fever, diarrhea and vomiting. If untreated, a person can die in less than 10 hours.

Paolo Basurto, UNICEF`s representative in Peru, said 50 percent of the country`s 22 million people lack access to safe drinking water, and two-thirds lack piped sewage. The UN estimates that 60 percent of Peru`s population lives in extreme poverty, and diseases such as tuberculosis and malnutrition are endemic.

Wracked by a vicious 10-year guerrilla war and a bankrupt economy, Peru`s public spending on social services has fallen by two-thirds since 1983, to $12 per person a year. The Health Ministry`s budget this year was slashed 60 percent.

''We had a health crisis before the cholera epidemic. This only made it worse,'' said Jose Luis Seminario, an epideminologist and member of Peru`s Cholera Commission.

Despite a health workers strike and the allocation of only $10 million to fight cholera, aid workers say the government responded well to the initial outbreak, launching an extensive television and radio campaign urging Peruvians to boil water, observe strict hygiene and avoid eating seviche and other uncooked foods. Officials also distributed more than 380,000 rehydration solutions, many donated by the United States and other countries.

But problems arose in March when President Alberto Fujimori, under pressure from Peru`s powerful fishing industry, ate seviche on television, saying Peru`s fish were cholera-free. His fisheries minister, Felix Canal, also ate seviche in a public display.

Health Minister Carlos Vidal resigned in protest. Canal reportedly contracted cholera. And the number of cholera cases rose sharply, with 19,892 cases and 126 deaths reported in the week following Fujimori`s exhibition.

Since then, new cholera cases have stabilized at about 4,000 a week, in part because the weather has cooled, slowing the bacterium`s growth. The death rate of those reportedly ill is less than 1 percent, a remarkably low figure compared with fatality rates of 15 percent or more for cholera victims in Asia.

''No poor country has the money to fight this disease,'' said Horacio Lores, a Lima-based epidemiologist with the Pan-American Health Organization. ''Unless there is massive help from wealthy countries, cholera will continue to spread. It will endure for years.''