"We go into the area where there's a nice little disease that belongs to the animals -- animals and sand flies, right?

"Okay," says Jorge Arias. "The first thing -- man goes in. He goes in with tractors. He goes in with bulldozers. He goes in with -- vroom -- what do you call them -- chainsaws. The first thing the animals do is say, 'Hey, let's go. Let's get out of here, okay?' And the sand flies stick around.

"You have disrupted something very serious in the sandfly -- its source of blood. And here comes this fresh source. Sleeps without a sleeping net. Sits around the campfire having a sip. Has to go to the bathroom in the tulies. And the next thing you know, the sandfly says, 'Hey, I need a source of blood.' And there you go. There goes man."

Arias, an animated Panamanian-born medical insect specialist, is an expert on the Amazon's single most effective weapon against human incursion -- disease.

The disease Arias knows best is called leishmaniasis. It is carried by sand flies, which pick up leishmania parasites from wild animals and pass them on when the flies bite human skin. The parasites give you skin ulcers that do not heal, and the only way to get rid of them is to undergo a protracted series of injections that make some people faint, grow nauseous or develop a kind of artificial arthritis.

Arias should know. He has had leishmaniasis twice.

Disease sets in, carried by jungle life or sometimes the colonist themselves, wherever men clear forest and build homes in Amazonia. Ninety percent of the area's population suffers from gastrointestinal diseases brought on by parasites and malnutrition, according to Roger Shrimpton, director of Arias' endemic disease program at the Manaus National Institute for Amazon Research. "Most people are loaded with parasites," he said.

Parts of southern Amazonia have some of the world's highest prevalencies of leprosy. African river blindness is traveling through parts of the Indian population along the In southern Amazonia's Rondonia territory, which is growing rapidly, 35 percent of the population has malaria, giving the territory one of the highest malaria rates in the world. northern perimeter, where ranchers and miners are eager to move in. And colonists and workers at the northeastern Amazonia hydroelectric project called Tucurui have brought in Chagass' disease, an incurable illness that is transmitted by bloodsucking insects and slowly kills its victims by affecting tissues in the muscle, heart, brain, intestine, liver and spleen.

"Malaria is the number one problem," Shrimpton said. In southern Amazonia's Rondonia territory, which is growing so fast that government officials cannot keep up with the influx of settlers eager for land, 35 percent of the population has malaria, giving the territory one of the highest malaria rates in the world, Shrimpton said.

"As soon as the road is tarmacked, it's going to be a hell of a problem," he said, referring to the new highway that is carrying most of the settlers into Rondonia. "These people come in from the south, and they have no immunity."

"The basic problems we have here are food, water, sanitation," said DeAnne Messias, an American nurse who works with project Esperanza, an American medical team that offers clinic services and travels the Amazon area by boat. "Our day to day struggles are with things like malnutrition, intestinal parasites, infectious diseases -- measles, whooping cough, pneumonias, skin diseases . . . measles is a very dreaded disease here. In the States, who worries about measles?"

Yellow fever, which was a serious Amazonia problem until the end of the century, is now negligible, Shrimpton said. Researchers at his institute are looking for ways to control malaria as well. There are indications they may one day provide an anti-malaria vaccine, use nutrition to build up immunity and help cut back mosquitos by using larva-eating fish.

But Messias keeps above her desk a small example of how far Amazonia is from any really adequate, widespread health care. It is a ribbon, one-third red, one-third yellow, and one-third green. The clinic gives them to Amazonia parents so they can wrap the ribbons around the upper arms of their children. If the ribbon meets at the red, it means the child's arm is so tiny that he is suffering from malnutrition and needs medical attention right away.