Researchers are unsure why human cases of the bird flu virus H5N1 gradually subsided amid fears of a human pandemic in 2007, reports STAT.
Four things to know:
1. H5N1 spread through Asia, North Africa and the Middle East, killing large amounts of poultry and 6 in 10 humans who contracted the disease.
2. Concerned about an H5N1 pandemic, health officials created emergency plans, stockpiled antiviral medications and worked on experimental vaccines against the bird flu virus. However, the number of human H5N1 cases waned on its own, and the last known H5N1 infection in a human occurred in February 2017.
3. Health experts do not know why the virus has faded over time but warn this trend could change.
"We don't know how the story going to end," Nancy Cox, PhD, a virologist who served as the director of the CDC's influenza division from 2006-14, told STAT.
4. Experts say the virus could mutate into different iterations that prove more infectious to humans.
"The H5N1 virus has not gone away. It's just changed into different versions of itself," flu expert Dr. Malik Peiris, a professor of virology at the University of Hong Kong, told STAT. "So now we have two high pathogenic [H5] viruses kicking around."