During the 2007-2008 influenza season, increased levels of resistance to the influenza drug oseltamivir (marketed as Tamiflu) were detected for the first time in the United States and worldwide. In addition, early 2008-2009 influenza season surveillance data suggest that oseltamivir resistance among influenza A(H1N1) viruses will most likely be higher, according to background information in the article. It was unknown whether some resistant viruses would cause clinical illness similar to other influenza viruses.