Federal and state medical investigators are in North Carolina this weekend looking into four rare cases of H1N1 Tamiflu resistant patients.

The seriously ill patients were all housed at Duke University Medical Center’s cancer ward in Durham.

They had been given Tamiflu to prevent them from getting ill, but recent tests over the past six weeks prove the virus mutated despite those treatments.

Three of the four patients died.

The Swiss-made Tamiflu is one of two medicines used in the U.S. to fight the pandemic.

Worldwide there have been 52 confirmed H1N1 resistant cases, with a cluster of five anti Tamiflu cases in Wales in the United Kingdom.