Thread by @kmpanthagani

Do not let anyone convince you that you need to get sick to be healthy. This is very silly.

While the day we fully understand the immune system will probably never come, there are a few things we can say for sure about how exposure to microbes impact your immune health...

1. Microbial exposures ARE important for immune development. For example, if you take away all the microbes from a mouse (germ free mice), their immune systems don't develop properly.

But when we say "exposure to microbes", we don't necessarily mean exposure to *pathogenic* microbes that make you sick. We often mean exposure to *commensal* microbes that help keep you healthy (i.e. the microbiome).

The reason germ free mice have messed up immune systems is not because they've never had a cold.

It's because we took away their microbiome.

Your microbiome consists of trillions of bacteria, viruses, and fungi that live on and inside you, regularly interacting with your immune system.

You don't need to get sick to be exposed to microbes. YOU ARE LITERALLY FULL OF THEM ALREADY.

2. Historically, pathogenic microbes were the number one killer of humans, especially children. In the year 1900, the top three causes of death in the US were all infectious diseases, accounting for a third of all deaths.

cdc.gov/mmwr/preview/m…

With improved sanitation measures, the discovery of antibiotics, and the development of vaccines, deaths due to microbes declined precipitously, and overall life expectancy grew BY THREE DECADES over the course of the century.

3. Viral infections can trigger autoimmune disease.

If you're hoping getting sick with a virus is going to "boost" your immune system and make you healthier, that is quite the gamble.

ncbi.nlm.nih.gov/pmc/articles/P…

4. Finally, things that are "natural" are not necessarily "good." This is the naturalistic fallacy, which attributes benefit to things that are naturally occurring, regardless of their actual consequences.

Infectious diseases are natural, but they aren't good for you.

Small correction: it's the "appeal to nature" fallacy, not the "naturalistic" fallacy.

(If only twitter had an edit button.)

In other news, MD/PhD student @IlanaOlin will soon be starting a microbiome fact vs fiction series on youcanknowthings.com, subscribe to follow along!

@IlanaOlin Will also add that certain microbes don't neatly fall into the "commensal" vs "pathogenic" category, some flirt with the line, depending on the context. The complexities of this are more than can be addressed here, but the bottom line remains: infections aren't good for you.

I'm seeing some people in the comments are confused about how adaptive immunity fits in, so here you go.

First, this thread was primarily directed towards the idea that we need repeated "stimulation" of our immune system with infections to be healthy and protected against allergies and autoimmune disease, etc.

This idea (sort of) derives from the hygiene hypothesis, which is the hypothesis that overly clean environments are bad for our immune systems and have led to the rise of allergic disease.

As new data has emerged, this hypothesis has evolved. It's not so much that you need to get sick... rather, data from the last decade has shown that exposure to microbe-rich environments is associated with reduced risk of allergic disease.

For example, kids who grow up on farms have a lower incidence of allergy and asthma, likely due (at least in part) to the (nonpathogenic) microbes and allergens they are exposed to.

For more details, read this by @minouye271: jacionline.org/article/S0091-…)

Think cows, not colds!

Now let's talk about adaptive immunity. This is the part of the immune system with which people are most familiar -- it allows your immune system to recognize *specific* pathogens.

If you get infected with a specific virus, your body will "learn" what that virus looks like and create a specific immune response to it. Sometimes that immune memory sticks around for a lifetime, and sometimes it wanes over time (like COVID).

Exposure to commensal microbes does not give you this type of specific immunity to pathogens. Your microbiome cannot train your adaptive immune system to recognize specific viruses like SARS-CoV-2, measles, rubella, etc.

So then does this mean exposure to these pathogens is "good" because you are training your immune system what they look like?

Well, yes and no... yes it's good that we have adaptive immune systems that can learn to recognize pathogens, but no it's not good to get sick. It would be better to never get infected in the first place (if that is an option).

If avoiding the pathogen all together is not a viable option, the next best option is to get the immunity without getting the infection. That's what vaccines do.