The Hygiene Hypothesis: True or False?

If you have a young child, you know that they spend a lot of time sick. “Their immune system is growing up just like they are, and it needs to learn what germs are,” says pediatric infectious disease specialist Frank Esper, MD.

But that raises the question: Could the things we do to keep our kids healthy actually hurt their immune system’s development? Some researchers used to think so, but it’s becoming clear that it’s more complicated than that.

Dr. Esper explains what the hygiene hypothesis is, how it’s changed over time and why protecting your kid against illness is work worth doing.

What is the hygiene hypothesis?

The hygiene hypothesis is a theory that being exposed to infections early in life trains your immune system, making it stronger later on. The theory also argues the reverse: That limiting exposure in childhood makes the immune system dysregulated and hyperresponsive, leading to autoimmune diseases and allergic conditions.

British epidemiologist David Strachan coined the term “hygiene hypothesis” in the late 1980s. He was one of several researchers who noticed that children born into large families with many siblings seemed to develop fewer allergies. Growing up on a farm also seemed to improve immune health.

Although the hygiene hypothesis emerged in the late 1980s and early 1990s, Dr. Esper says the ideas behind it date back to the 1960s.

“We know that it’s more complicated now, but back then, researchers observed that communities in certain areas of Africa didn’t seem to have rheumatoid arthritis at all,” Dr. Esper shares. “They wondered if repeated exposure to malaria and other viral and parasitic infections was protecting those communities against inflammatory conditions.”

The hygiene hypothesis was one way to explain the increase in autoimmune and allergic diseases in industrialized areas with strong sanitation and public health systems. Think conditions like:

Is it a myth or a fact?

As of now, the hygiene hypothesis is exactly that: A hypothesis. It’s been neither proven nor disproven.

“The problem is that an association does not mean causation,” Dr. Esper notes. In other words, studies show that there’s a pattern, but they can’t explain why the pattern exists.

The “why” is a controversial subject, but a growing body of evidence collected over the past 25 years suggests early-onset infections don’t protect against immune issues later in life. That makes sense because we know getting sick with viruses — like respiratory syncytial virus (RSV), COVID-19, and even the common cold — doesn’t protect us against reinfection or (in the case of chickenpox) reactivation.

Thanks to that research, Dr. Esper says the theory has shifted. Today, the focus is less on viruses and more on the bacteria that make up our microbiome.

“The good bacteria in your gut help you digest food and train your immune system to tell the difference between good bacteria and bad bacteria,” he explains. “The current thinking is that changes to food safety and dietary habits have altered our microbiomes and reduced our exposure to certain kinds of bacteria, causing a decrease in early childhood infections.”

Of course, fewer childhood infections also means fewer children dying of preventable illnesses. That’s a good thing! If the hygiene hypothesis is right, the rise in inflammatory and allergic conditions may be the price we have to pay for progress — at least for now.

Dr. Esper is hopeful that we’ll eventually untangle the relationship. And once we understand what’s causing the rise in immune system dysregulation, we’ll be able to address it.

“I think we’ll probably figure it out,” he adds. “But it’s going to take more studies to get there.”

How to build your child’s immunity

Dr. Esper emphasizes that the hygiene hypothesis isn’t a reason to be less hygienic or to take fewer steps to prevent your child from getting sick.

“We’re trying to move away from the term ‘hygiene hypothesis’ because we don’t want people to think they need to get infections to be safe,” he emphasizes. “There’s nothing good about getting the flu. So, we don’t want you to go out and try to get your kids sick, thinking that they’ll get sick later in life if you don’t.”

Getting sick is an important part of growing up. But that doesn’t make preventing illness less important. Dr. Esper says the best way to build your child’s immunity is to:

• Vaccinate them. Vaccines are a safe and effective way to train your child’s immune system to recognize and defend against dangerous illnesses.
• Teach them proper hygiene. From hand washing and cough etiquette to nail care and proper brushing technique, good hygiene can help your kiddo limit unnecessary exposure to germs.
• Feed them a balanced, varied diet. That means lean proteins, fresh fruits and vegetables, whole grains, healthy fats and foods that are rich in pre- and probiotics. Teaching your children about nutrition will also set them up for a healthier tomorrow.
• Limit their processed food intake. Avoid heavily processed foods, which tend to include added sugars, saturated fats and refined grains.
• Get them moving. When kids run and play, they push oxygen-rich blood throughout their bodies, speeding recovery. That blood’s also full of immune cells that detect and prevent infections.
• Protect their sleep. Little bodies need lots of consistent, quality sleep to strengthen their immune system. It’s also crucial when recovering from illness.
• Consider supplements. If your kid seems to be getting sick more often than other children their age, ask their pediatrician about adding vitamin D and zinc supplements to their routine.

Bottom line? We still have a lot to learn about the immune system, but if you’re doing what you can to keep your kiddo going strong, the current science says you’re on the right track.