The US Centers for Disease Control says the flu virus has already reached epidemic levels in the US, raising concerns.
It is now widespread in 36 states cross America. The flu has taken the lives of at least 15 children so far. Health officials say people older than 65, children under 5, pregnant women and people with medical conditions are the highest risk.
Part of the problem this year is the flu strain mutated in ways which makes the flu vaccine less effective than experts hoped. Flu activity usually peaks between December and February, but it can last until as late as May.