ATLANTA — Some swine flu cases in Michigan are raising questions about obesity's role in why some people with infections become seriously ill.

A high proportion of those who have gotten severely ill from swine flu have been obese or extremely obese, but health officials have said that might be due to the fact that heavy people tend to have asthma and other conditions that make them more susceptible. Obesity alone has never been seen as a risk factor for seasonal flu.

But in a report released Friday, health officials detailed the cases of 10 Michigan patients who were very sick from swine flu in late May and early June and ended up at a specialized hospital in Ann Arbor. Three of them died.

Nine of the 10 were either obese or extremely obese. Only three of the 10 had other health problems. Two of the three that died had no other health conditions.

This hardly settles the question of whether obesity is its own risk factor for swine flu. It's possible the patients had undiagnosed heart problems or other unidentified conditions.

Still the finding was striking, investigators acknowledged.

Also remarkable were that five of the patients developed blood clots in their lungs, and six had kidney failure. Those complications have been seen in some swine flu patients before, but not usually in such a high proportion.

"Clinicians need to be aware that severe complications can occur in patients with the novel H1N1 virus, particularly in extremely obese patients," said Dr. Tim Uyeki, a flu expert at the Centers for Disease Control and Prevention.

Uyeki was a co-author of the report, released by a CDC publication, Morbidity and Mortality Weekly Report.

Also on Friday, the CDC said the number of U.S. swine flu cases has surpassed 37,000 and deaths have risen to 211.

The numbers rose from the 170 deaths and nearly 34,000 confirmed and suspected swine flu cases reported last week.

Those are lab-confirmed and probable infections. CDC officials believe those cases -- which sought treatment and underwent testing -- are just the tip of the iceberg. They estimate more than 1 million Americans have been infected with the virus so far, though many probably had only a mild illness.

Swine flu is the predominant flu type circulating currently, with nine states reporting widespread cases, down from 10 a week ago.

The pandemic was first identified in California in April. Since then a total of more than 94,000 cases have been reported in more than 100 countries, according to the World Health Organization.