Bird flu (also called avian flu or avian influenza) is an infection that occurs naturally in birds. Human cases of bird flu are rare, but are becoming more prevalent in Asian countries. So far, research suggests that two antiviral medicines, oseltamivir (Tamiflu^®) and zanamivir (Relenza^®), may be useful for H5N1 bird flu. However, H5N1 viruses are generally resistant to two other available antiviral medications, amantadine and rimantadine, so they cannot be used to treat bird flu. Vaccines to protect humans against the H5N1 strain of bird flu are currently under development, as is research to make large quantities of vaccine more quickly.