India, Dec. 20 -- In January of 2025, three tigers and a leopard died at the Wildlife Rescue Centre in Nagpur within a short period of each other. Their deaths were due to an influenza virus that normally infects only birds, the H5N1 subtype of the influenza A virus. The term 'bird flu' applies generally to influenza infections in birds. H5N1 is one example, but not the only one.

In general, viruses are specific to species, although there are a few notable exceptions, such as the rabies virus. Viruses capable of infecting species that are very different from the original host must undergo a 'spillover event', in which a mutation enables them to survive and adapt to the new host. Such events are relatively rare.

H5N1 does, in fact, infec...