When it comes to avoiding illness, hand-washing is one of your first lines of defense. While good hygiene is certainly important, is there such a thing as washing too much? Not really.

The concept that not washing your hands will help you build up your immune system was introduced in the late 1980s by a professor who suggested that not exposing yourself to bacteria could compromise your immune system, making you susceptible to illness. The theory suggested that exposure to small amounts of bacteria "trains" your immune system to fight germs. This training then strengthens your immune system, making it better able to respond when more germs invade your body.

The reality is, washing your hands is vital to keeping yourself healthy and avoiding the spread of illness. The flu virus can live on a door handle for 24 hours. Washing your hands can prevent one in three diarrhea-related sicknesses and one in five respiratory infections. This 20-second routine can single-handedly help you in the battle against germs, including coronavirus, the virus that causes COVID-19.

A good rule of thumb is to wash hands thoroughly when needed, such as prior to handling food, after using the restroom and after blowing your nose. To most effectively remove germs, the Centers for Disease Control and Prevention (CDC) suggests washing your hands for 20 seconds, about the time it takes to sing "Happy Birthday" twice. Dry hands completely afterward to remove any lingering germs.

If soap and water are not available, you can also use a hand sanitizer. Make sure to buy a product that is at least 60% alcohol.

• Northwestern Medicine Infectious Disease Physician Luis A. Manrique, MD