Virus particles are much smaller than human cells, about a millionth of an inch smaller. That means they can reproduce quickly and cause many serious complications. But these viruses can't live on their own. They need a host to carry out their life mission: to infect and replicate.
Coughing is your body's natural immune response as it tries to fight viral invaders. It's also one of the most common ways tiny droplets of flu-containing saliva and mucus infect your respiratory lining.
"Influenza causes typical symptoms of fever, body aches. You just kinda feel lousy," said Dr. Rekha Murthy, the head of epidemiology at Cedars-Sinai Medical Center.
Dr. Murthy says you feel lousy because when the virus feeds off the cells in your nose and throat, it starts to reproduce wildly. Your body's temperature rises to try to stop the replication. The symptoms that follow are the virus spreading to your lungs, bloodstream and muscles.
"The influenza illness causes a pretty overwhelming illness, and not just the fever, but it really shuts down a lot of the immune system," said Dr. Murthy.
And when you have a weakened immune system, secondary bacterial infections can set in and attack your lungs and different organs.
"If you're having symptoms of dehydration, if you're feeling dizzy or confused, or if you're having nausea and vomiting and can't keep food down, those are the kinds of situations that you should at least call your doctor," said Dr. Murthy.
Immunizations work by pre-infecting the body so it knows how to produce the right antibodies as soon as the virus starts reproducing. The flu bug undergoes slight changes from season to season. This is why new vaccines must be produced constantly and given yearly.
As Dr. Rekha Murthy reminds us, nobody is immune.
"Even though there are more complications of death, et cetera, in the elderly and the very young and those with underlying conditions, it can happen in healthy individuals as well," said Dr. Murthy.
Research shows the influenza virus can live up to eight hours on most surfaces.
While there are few studies that show how fast flu spreads by sharing cellphones, experts say you shouldn't share your phone with someone who might be sick, especially since the virus would only have a short distance to get into your system from the phone to your nose or mouth.