When we say “the Flu,” we mean Influenza. Influenza A and B are viruses that occur from mid-November to early May in the U.S. Outside of this “flu season,” other viruses can cause the same symptoms (see Differences Among Germs). Influenza doesn’t cause vomiting or diarrhea; even though we call that a “stomach flu,” it’s a completely different virus.
Influenza begins abruptly with fever and body aches. Cough occurs soon after. Fever goes away on the 3rd or 4th day, everything else gets better soon after, though some people may remain fatigued for another 1-2 weeks.. Any other pattern means it’s not the Flu. Or, it could mean a complication, like bacterial Pneumonia that can develop when things seemed to be getting better. Those at most risk are elderly, young babies, pregnant women, and people with other conditions (mainly heart or lung disease).
Physical exam is normal with Influenza. Chest x-rays are also normal, although we don’t usually order them. A rapid test from a nasal swab is very accurate if positive, but has 40% false-negatives.
It’s likely impossible to tell the difference between Covid-19 and Influenza. We’d never diagnose the Flu without doing a Covid test. Even if the Influenza test is positive, there are cases of people having both infections together.
There are anti-viral medications that can shorten the illness by 1-2 days, if given within 48 hours of symptoms. They don’t prevent complications in healthy people, but may among the elderly and those with chronic illnesses (heart & lung disease, liver or kidney failure, Diabetes, Sickle Cell, extreme obesity, etc.). Otherwise, it’s lots of fluids, chicken soup, & a few days off work. Since Influenza very contagious, it’s important to cover your mouth when you cough, and wash your hands often. Wearing a mask may stop spread, but is unlikely to prevent getting the virus.
Best strategy of all is vaccination. Vaccines change each year depending on which strains the experts think will be around. Vaccines are fairly good at protecting against getting the Flu, but are especially important because they protect you from dying of it. They become available around September, and should be given before November at the latest. Our main fear is that a completely new strain might appear, like the one that so far is only spread from bird to bird. Bird Flu is very dangerous to humans, but the only cases so far have been a very few people in Asia who kept chickens.