Flu often causes all sorts of nasty complications, from lung infections (sometimes full pneumonia), encephalitis, heart inflammation, muscle inflammation, etc.etc. It can actually hit your nervous system too, a have a surgeon friend who had a partial paralysis of one of her hands after going through the flu. It’s a serious illness with serious complications and should never be taken lightly. This is why your doctor may actually prescribe you antibiotics when you’re going through the flu, even though they cannot cure the flu itself, as it is a virus and antibitiocs have no effect on it, but they do have an effect on the bacterial complications it leads to.