Here are two fears that Americans seem to have in the wake of the discovery of mad cow disease in a Washington cow, and the science of assessing them is very different.

The first: Did my family eat any of that cow, and, if so, will it hurt them?

The second: Never mind that one cow — how many others are out there?

Answering the first is really a matter of looking at the history of similar brain diseases in Britain and New Guinea.

Answering the second is, for the moment, largely a matter of statistics — but difficult, because the numbers are so vague.

It seems almost inevitable that some part of the cow was eaten. It was killed on Dec. 9, and ground up with about 20 others to make a batch of 10,000 pounds of hamburger that was shipped to groceries in eight states and Guam, although 80 percent went to Oregon and Washington, the Agriculture Department says.

The diseased cow was not found until Dec. 23, and a recall order was issued.

Dr. Gary Weber, a spokesman for the National Cattlemen's Beef Association, said he thought that, like most ground beef, the batch would have been frozen for transit. He had heard that 20 percent was found in storage, he said. But he said of the rest: "I'd hazard a guess that some of it has been consumed."

Nonetheless, even if the meat was eaten, the risk to humans seems low.

Dr. Ron DeHaven, chief veterinarian of the Agriculture Department, has described it as "minuscule, if any risk at all."

In Britain, nearly 200,000 cows were infected. Millions of people ate meat from those cows, including steaks, ribs, hamburger, neckbones, beef marrow and brains. Material from cows was used in a wide variety of items, including beauty products, polio vaccines and weightlifters' steroid substitutes. A lion in the Newquay zoo in England was found to have a form of the disease. Yet only about 150 Europeans have died of it. Early predictions of 100,000 to 200,000 British deaths did not come true.

Some research has indicated that not everyone is equally at risk, that some people have a genetic predisposition toward the disease.

Moreover, assuming the Agriculture Department was correct, and only muscle meat from the Washington cow was ground up, the risk is probably far lower. Although prions, the misfolded proteins that cause the disease, have been found in the muscles of hamsters, mice and humans infected with the disease, brain and nerve tissue is thought to be a million times more infectious.

But not all scientists agree. When young animals are infected, the disease does not show up in their brains for at least 30 months, said Dr. Stanley B. Prusiner, a neurologist at the University of California in San Francisco who won a Nobel Prize in medicine for his work on prion diseases. But it could be present in low levels in other tissues, including muscle, Dr. Prusiner said, and at higher levels in the lymph glands of calves. It is not clear that those levels would be enough to infect anyone, and federal officials have asserted that the muscle meat is safe.

Also, it is possible for brain tissue to be driven into or splattered on muscles in a slaughterhouse. Animals are usually killed with a blow from a hand-held jackhammer that slams a piston into the skull. The last few beats of the animal's heart can circulate the tissue. Also, sawing a carcass in half can splatter spinal cord tissue around, as can the use of high-pressure jets that strip meat from bone. Agriculture officials said they believed that such methods were not used on this carcass.

Dr. Robert L. Klitzman, an expert in prion diseases at the school of public health at Columbia University, who in the 1970's studied a brain-wasting disease known as kuru that nearly wiped out the Fore tribe of New Guinea, found that even some people who had eaten the brains of other humans at cannibal feasts did not become infected.

Moreover, Dr. Klitzman said, many of the Fore who were infected with the prion-caused disease may have become so not from eating, because digestion breaks down most proteins into amino acids, but because they did not wash after a feast, and may have put it directly into their blood by scratching insect bites.

"The oral uptake is not very good," he said. "That's why there was not much of an epidemic in Britain."

On the larger question — how many other cows have the disease — Dr. DeHaven said the department believed that "the worst-case scenario is the disease exists in the United States at a very low prevalence."

Dr. Klitzman said it "would not surprise me at all if we have more." But he said the chances of two cases arising spontaneously were remote. He said the two cows could have been infected from one source of feed.

Until May, Canada and the United States formed one large beef market with animals freely crossing the border. That border was closed in May after Canadian agriculture officials found one case of mad cow disease in a beef steer, but many scientists warned then that any cattle disease found in Canada was likely to be found in the United States, too.

The possibility that both cows were born in Alberta — although Canada is not ready to concede this — can be read as encouraging news, scientists said. It suggests that the infection might have one source, and might be a regional problem rather than a continental one.

Dr. Donald Berry, chairman of the biostatistics department at the University of Texas's cancer research center, while cautioning that he was not an expert in mad cow disease, said that finding two positives in roughly 40,000 recent tests would suggest that there could be about 1,750 positive animals in the 35 million slaughtered each year.

But Dr. Berry pointed out that there were many variables. For example, the Agriculture Department says that most tests are of "downer" animals, many of which are old and therefore much more likely to show signs of the disease, which takes four to six years to incubate. Older animals are a small part of the overall annual slaughter, and downers are a small number of that group.

If both positives turn out to have been from cattle born in Alberta, even if the one detected in May was a Black Angus cross steer and the Washington one a Holstein dairy cow, it makes it "more likely that it's isolated and comes from one area," Dr. Berry said. "The feed could have come from one feedstore."

Critics of the beef industry and the Agriculture Department remain sure that more diseased animals exist.

Agriculture Department officials said their testing was intended to spot the disease even if it appeared as rarely as in one of one million cattle.

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