Why you keep getting sick, according to infectious disease experts

• Most adults catch a common cold two to three times each year, according to the CDC.
• Infectious disease experts explained why someone might keep getting sick.

Most adults catch a common cold two to three times each year but there are two reasons why you might get sick more often than average, according to experts.

Experts told Insider that, generally, the number of times an adult gets sick varies according to: how often they're exposed to a virus and their immune system.

"People vary a lot," Otto Yang, a professor of medicine in the division of infectious diseases and of microbiology, immunology and molecular genetics at the David Geffen School of Medicine at UCLA, told TODAY.

"There are people that feel like they're sick the whole cold and flu season and other people that seem to be untouched by it," he said.

Common cold symptoms — which can last around seven to 10 days — include: a cough, sore throat, runny nose, congestion, headaches, and sneezing. Many different viruses cause it, though rhinoviruses are the most common.

You have kids or look after them

Kids get sick more frequently than adults and those who have kids — or look after kids — often catch it from them.

Thomas Murray, a pediatric infectious disease physician at Yale New Haven Children's Hospital, told Insider that kids are more likely to get sick because their immune systems aren't fully developed and they spend more time in crowded places with other children who get easily infected too.

Preschool-aged kids can get sick up to once a month but this number reduces to about four to six times per year once they get older enough to practice hand hygiene, Dr. Frank Esper, a pediatric infectious disease specialist at the Cleveland Clinic, told TODAY.

You're exposed to many other people at work

Murray, also an associate medical director for infection prevention at Yale New Haven Children's Hospital, said that where a person works and their behavior can contribute to the likelihood of getting sick. For example, generally people are less likely to become infected while working from home than in a job where there is contact with multiple people in a day.

People who don't wash their hands and stay close to people with symptoms are more likely to get infected, he added.

Paul Hunter, a professor in medicine at the Norwich Medical School at the University of East Anglia, UK, told Insider that commuting to work on a crowded train can also make it easier for cold viruses to spread.

Colds are caused by a 'huge range of viruses'

Once you're exposed to a cold virus, it is up to your immune system to fend it off.

One of the reasons colds are so common is that they're caused by a "huge range of viruses," and catching one doesn't mean you're protected against another, Hunter, who specializes in diagnosing, treating and preventing infectious diseases, said.

If you get sick today, then you will have some protection from another infection for about a week afterwards because of a protein called interferon, which is part of the body's defense against viruses, Hunter said.

Murray said that for most adults who get frequent colds lasting a few days, they're probably experiencing all the viruses that circulate in our environment and "the natural course of being out in public and social again" post-pandemic.

However, colds lasting more than 10 days or bacterial infections on top of the cold that don't respond to antibiotics can be signs of a weak immune system, either caused by a health condition or medications, he said.

The CDC states that people with colds that are "severe," "unusual," or last more than 10 days should see a doctor.

You can't prevent colds entirely

Hunter said that there is some research to suggest that vitamin D supplements can help protect against respiratory infections, particularly for those older than 60.

However, generally there isn't much that people can do to prevent a cold because the viruses that cause them are so infectious.

"If you think you can prevent yourself from getting colds, you're mistaken," he said.