(NEXSTAR) – Flu cases are surging across the country – more than half of states are now at "very high" levels, the most severe designation by the Center for Disease Control and Prevention – but is it possible that some of those people are catching it for a second time?

Flu is an illness caused by influenza viruses that often arrives suddenly with symptoms of fever, cough, headaches, sore throat and more. "Flu can cause mild to severe illness, and at times can lead to death," according to the CDC.

There are four types of flu (A, B, C and D), but the annual "flu season," typically starting in October and lasting as long as

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