The convergence of multiple flu strains in one animal can allow new virus strains to form—just as they did in past pandemics.

As bird flu continues to spread through dairy cow herds and poultry flocks, the Missouri case has raised the possibility of human-to-human transmission of bird flu. But officials stressed Thursday there is no evidence of other people being infected and the risk to the general public remains low.

A person sickened by bird flu in Missouri was probably an isolated case, federal health officials said Thursday, though they are still investigating how the person became infected with no clear source of transmission.