Experts believe H5N1 bird flu belongs in a growing category of infectious diseases that can cause pandemics across many species. But there are ways to reduce the risks

Bird flu poses a threat that is “unique and new in our lifetime” because it has become a “‘panzootic” that can kill huge numbers across multiple species, experts warn. For months, highly pathogenic bird flu, or H5N1, has been circulating in dairy farms, with dozens of human infections reported among farm workers. It has now jumped into more than 48 species of mammals, from bears to dairy cows, causing mass die-offs in sea lions and elephant seal pups. Last week, the first person in the US died of the infection.

This ability to infect, spread between, and kill such a wide range of creatures has prompted some scientists to call H5N1 a “panzootic”: an epidemic that leaps species barriers and can devastate diverse animal populations, posing a threat to humans too. As shrinking habitats, biodiversity loss and intensified farming create perfect incubators for infectious diseases to jump from one species to another, some scientists say panzootics could become one of the era’s defining threats to human health and security.