The US is entering a riskier season for spread of H5N1 bird flu. Here’s why experts are worried

Fall and winter months present more opportunities for H5N1 to spread and change, officials say.

Each new host gives the virus a chance to get better at infecting people, experts say.

The CDC is trying to sequence the rest of the virus’ genome to find out whether it is related to those infecting poultry and dairy cattle.

Only 13 other human infections in the US this year have been reported in humans.

The first infections of cows with the H5N1 virus were confirmed in Texas and Kansas in late March .

Evolutionary biologists think cows became infected months earlier , perhaps around the first of the year .

The finding that the virus infecting the herds in California is genetically related to these first infections is important because it means they almost certainly are not the result of another crossover event.

The government has ordered that nearly 5 million doses of H5N1 vaccine be packaged and made ready for use.

There are no plans to actually give these doses to anyone, including farmworkers, who are most at risk.

This policy stands in contrast to Finland , which has seen past H5n1 outbreaks on fur farms.