During a call with U.S. doctors this month, one CDC official noted that there is a seasonality to bird flu: Cases peak in the fall and early winter, possibly due to the migration patterns of wild birds that are primary spreaders of the virus.

Bird flu has killed over 166 million birds worldwide and is now spreading to other animals, including cows, and more concerningly, cats.

Over a 14-month period, 70 people in the US were confirmed to have bird flu. But since early February, the number of new diagnoses stands at zero—leaving experts to question why reports of new human cases have essentially stopped.

The first major global review of bird flu in cats reveals an emerging threat of a potential human pandemic. Spring is here, birds are on the move, and a new threat is spreading alongside them: bird flu (H5N1).