Researchers have recently cracked a portion of the avian flu puzzle. Flu viruses can bind to two different receptors in human respiratory tissue: a small cone-shaped receptor and a long umbrella-shaped receptor. So far, H5N1 human cases have been the result of viruses attaching to the cone-shaped receptor in the upper respiratory tract. To date, the H5N1 virus has not been able to attach to the umbrella-shaped receptor. However, if the virus mutates in such a way that its tropogens can link up with these receptors, human to human transmissibility is very likely. The most contagious human flu viruses in the past have been able to bind to the umbrella-shaped receptors. This latest discovery will likely help to improve surveillance and to open doors to new drugs.