Though it may not be pleasant to think about, countless microbes live on and in our bodies, and they are actually necessary to maintain good health. 

“Good bacteria in our body provides us with nutrients we need and also provides a defense against bad bacteria and infection,” Tran says. A balance needs to be maintained because when good bacteria is reduced, bad bacteria can take over, making us feel sick, he says. 

Stress can lead to increased levels of cortisol , a steroid hormone that is important for overall function of our body; but too much of it can lead to a number of health problems, including decreased immunity. “The high level of steroids can blunt your immune system,” Tran says.

Some research suggests that optimism can actually make our immune system work better. “I don’t know if it’s a direct cause,” Tran says, “but the happier, or more positive you are, the more likely you are to eat right and be less stressed , which will help your immune system.”

Not getting enough sleep can wreak havoc on the body, and the immune system is no exception. Studies show that a lack of sleep may make you more likely to catch a cold and also makes it more difficult to fight off infection.

When you experience an allergic reaction , your immune system is responding to a harmless allergen that it perceives as a threat. Symptoms of an allergic reaction, which can range from a runny nose to breaking out in hives to fainting, are a result of the body’s misguided attack.

Autoimmune diseases occur when the immune system destroys its own healthy tissues. In such cases, white blood cells in the body cannot distinguish between pathogens and the body’s normal cells, setting off a reaction that destroys healthy tissues.

While there are over 80 different types of autoimmune disorders, common ones include rheumatoid arthritis, psoriasis , and Crohn’s disease .

Autoimmune diseases affect about 8 percent of the population in the United States, making up the third most common category of disease after cancer and heart disease, according to the U.S. Centers for Disease Control and Prevention . Women make up nearly 80 percent of those living with autoimmune diseases. While the cause of autoimmune diseases is unknown, it is generally believed to have a strong genetic component, and women in their child-bearing years are at the highest risk.

Cleaning and disinfecting may seem like the best way to avoid infection, but this is a case where there can be too much of a good thing.

“When you make your environment so clean, you minimize so many foreign pathogens that you actually minimize the development of the immune system,” Tran says. This is especially the case with young children, since if they’re not exposed to harmful microbes at all, their bodies won’t be able to develop the proper antibodies to fight them off.