Two people with compromised immune systems who became ill with 2009 H1N1 influenza developed drug-resistant strains of virus after less than two weeks on therapy, report doctors from the National Institute of Allergy and Infectious Diseases (NIAID), part of the National Institutes of Health. Doctors who treat prolonged influenza infection should be aware that even a short course of antiviral treatment may lead to drug-resistant virus, say the authors, and clinicians should consider this possibility as they develop initial treatment strategies for their patients who have impaired immune function.