Additionally, taking those antibiotics may lead to infections that are resistant to those medications.
"Bugs are very smart," Brownfield said.
When to See A Doctor: If you have trouble breathing or feel pain in your chest, or if you have a persistent fever, it may be time to pay your physician a call.
"I would seek medical attention, if at any, any point, people are confused or dehydrated," said Brownfield.
Avoiding It: Pneumonia, like influenza, has a vaccine shot. Unfortunately, like influenza, the vaccine doesn't protect against every strain. Additionally, pneumonia can be caused by bacteria, which a vaccine does not protect against.
Good hygiene, like handwashing, can help you avoid it. Also, if you have flu, make an extra effort to avoid sick people, as you will be more susceptible to pneumonia.
If You Get It: Pneumonia, like bronchitis, affects the lower respiratory tract. Many people, when they cough up mucus, will see it's yellow or green -- but even if you don't see that color change, you might still have this illness.
To Feel Better: Pneumonia isn't something you should try to recover from on your own.
When to See A Doctor: Only an X-ray can confirm for certain if you have pneumonia or bronchitis. If you find you are having trouble breathing, pick up the phone.
Avoiding It: "Stomach flu is a misnomer -- it's not caused by any flu virus, it's caused by a different type of virus," said Brownfield.
Schaffner explained that stomach flu tends to have an uncertain cause, so aside from general hygiene, it's difficult to pinpoint a way to avoid it.
If You Get It: The virus will often run its course in 24-72 hours, so you should recover much more quickly than you would from influenza.
While you have stomach flu, however, you can expect nausea and, occasionally, vomiting, but more frequently bouts of diarrhea.
To Feel Better: "The most important thing I would once again emphasize is fluid intake," said Schaffner.
Because the disease tends to make you lose fluids, he recommends small sips of fluid, including a carbonated beverage like ginger ale or cola, that will soothe your stomach.
However, Schaffner said, you don't want to drink too quickly, as that may induce nausea or even vomiting. Instead, you want to keep yourself hydrated in a "gentle and persistent fashion."
When to See A Doctor: You should see a doctor if the diarrhea persists, if you feel any kind of abdominal pain or if your stool has blood in it.
If any of that happens, said Schaffner, that's the time to call your doctor, visit him or her, or go to the ER.
Cold & Flu season is here! Visit the ABCNews.com OnCall+ Cold & Flu Center to get all your questions answered about these nasty viruses.