A rare intestinal illness contracted by 68 people across the country so far this year may be linked to undercooked hamburgers, according to the Centers for Disease Control.

A CDC spokesman said yesterday that federal and state health officials linked outbreaks in Oregon and Michigan to the consumption of hamburgers from McDonald's restaurants in two cities. Twenty-six people became ill in February and March in White City, Ore., and another 21 in Traverse City, Mich., in May and June after eating at the popular chain restaurants.

Since then, said CDC's Donald Berreth, another 21 "sporadic cases" have occurred around the United States in people eating at a "variety of fast-food restaurants."

There have been no fatalities, but the diarrhea-illness was serious enough to hospitalize many of the victims. Most recovered within a week.

"The presumption is that something was wrong with the meat. People need to be aware that there are illnesses transmitted through hamburger that wasn't adequately cooked. But it's a very rare disease. Nobody is thinking of suggesting that you stop eating hamburgers," said Berreth.

The McDonald's Corp., based in Oak Brook, Ill., contended that the link with its hamburgers might be a statistical anomaly and said that the company's required cooking procedures "ensure product safety."

News of the scientific findings created a flurry of activity on the stock market Thursday, leading to a suspension of trading in McDonald's shares and a drop in the price. After dropping a point to 56 Thursday, the stock rose yesterday to a closing level of 60 1/4, suggesting that statements by the company and the government had successfully stemmed a panic reaction.

"You bet it's safe to eat at McDonald's," said company Vice Chairman Edward H. Schmitt, in a vigorous defense of his company's leading product. He said that the chain had served 2 billion hamburgers since these "isolated instances" were first reported and that its customers receive "the highest quality product you can buy."

Two CDC researchers, Dr. Mitchell Cohen and Lee Riley, reported at an American Society for Microbiology meeting in Miami this week that the intestinal illness, called hemorrhagic colitis, was apparently caused by a form of the bacterium Escherichia coli.

The organism, which has not previously been linked with human disease, can be killed by cooking food thoroughly at high temperatures, said Berreth. "One would assume that thoroughly cooking the hamburger would solve this problem," he added.

Berreth suggested that the source of the bacterium may be a "relatively rare disease in cattle and might be confined to only a few places." But federal investigators have not traced the meat.