An investigation is underway after the discovery of four Tamiflu-resistant swine flu cases at a North Carolina hospital.

"USA Today" reports that health researchers at Duke University Medical Center in Durham are trying to find out if the resistant strain has started spreading inside or outside the hospital.

If that's the case, it's a first for a Tamiflu-resistant virus to be transmitted from person to person.

The four patients affected were in a ward for patients with cancer or severe blood disorders. All were reported to be severely ill and highly susceptible to infections.

The patients got the flu last month. When Tamiflu treatments failed to help them get better, the hospital had samples of their virus tested. The results came back two weeks ago, and Duke started its investigation.