English-language werewolf novel from the mid-to-late 1980s

The book begins much as you described. An anthropologist is listening to members of a primitive tribe talk about how a werewolf had been killing people, twelve years ago, in sprees which always occurred during a full moon, until the tribal shaman was able to come up with a way to kill the monster. The scholar is interested in letting them talk, but personally suspects the "werewolf" was something like a victim of rabies who foamed at the mouth and seemed inhumanly strong as he went on a rampage -- although it is odd that he would survive long enough to launch three killing sprees at monthly intervals. The scholar silently speculates that perhaps there were multiple rabid attackers, in a monthly cycle -- each new one having been infected by the previous before he died of his disease, and only the last of them was somehow trapped and killed by the men of the tribe, who then blamed this "monster" for all of the previous killings.

The scholar is trying to be polite, but evidently his skepticism shows through. The shaman is offended at being patronized by a white man who thinks he already knows all the answers. The shaman had kept a souvenir of the werewolf, dried out and in powdered form, believing it still contained the power to create another werewolf if need be. The shaman (called "Ugalde") puts that special powder into a bottle of whiskey and then . . .

With the swiftness of a man half his age, the Indian leaped from a sitting position onto the other's chest, as if he were pinning him in some friendly athletic contest. But the crazed expression covering his face and the keenness of the knife blade removed any trace of ordinary competition from the action. Ugalde sat the bottle viciously on the ground next to the visitor's face and tipped the neck of it toward his mouth. "Drink," he ordered.

"Ugalde!" the man cried frantically. Visions of wriggling microbes being revitalized by the liquor flooded his mind. Could any life survive for a dozen years in such a dehydrated state? Yes, yes, viruses could live for centuries! "What in the name of God are you doing?"

A sneer replaced the sullen look the older man had worn. "Your god! You believe your eye, white man, and only that, so drink and see what the devils you don't know can cause a living spirit to become! Drink, or I will cut your throat!"

Then a good chunk of the book deals with brutal murders taking place, always on the night of a full moon, and various characters getting interested in the investigation, for various reasons. At first, there's a serious lack of surviving eyewitnesses to describe the mysterious attacker to the cops, but gradually the evidence accumulates that something strongly resembling a Hollywood-style werewolf is involved in these homicides. We readers, of course, have a very good idea of how that happened.

Then another large chunk of the book deals with what happens in one horrible night after the anthropologist has been taken into custody. (I think he finally surrendered himself to the police, voluntarily, and described everything he could remember about the incident with the shaman, and what's happened since then on the nights with full moons in the sky.) You are correct that becoming a werewolf appeared to be a type of communicable disease, although I can't recall if it was specifically established, during the novel, to be a "germ" or "virus" or what. You are also correct that silver was particularly effective against a werewolf. Other weapons could pierce its flesh, but it appeared to recover very rapidly from such wounds. (Sort of like Wolverine's mutant "healing factor" in the X-Men comics and movies.)