Thread by @EpiEllie

Are respiratory infections secretly preventing cancer? No.

Do small amounts of debris from certain bacteria help prevent allergies & fight cancer? Maybe?

Does infection with certain bacteria & viruses *cause* cancer? Yes, that's a fact!

Infections & cancer, a thread🧵

First, let's be clear: not every bacteria or virus that people can be infected with causes cancer! I'm not saying that at all.

But some bacteria and some viruses *are* known to cause cancer. Let's talk about some of them.

First up: human papillomavirus, or HPV, which causes many different cancers: Cervical cancer, penile cancer, anal cancer, oropharyngeal cancer

HPV transmits by sex so just about any body part you use for sex can get HPV. And if it can get HPV, it can get cancer.

HPV was widespread before we had a vaccine, and most sexually active people were probably exposed at some point. But not all infections lead to cancer.

Most of the time, our immune systems fight the infection easily. But when we can't, HPV damages our cells & causes cancer.

If that sounds kinda scary, then I've got great news for you:

We have a really effective vaccine against HPV! There's even one that protects against 9 different strains (think of it like a multi-variant covid vaccine).

Thanks to the HPV vaccine, cervical cancer rates are way down!

But because the vaccine was first targeted only at girls and young women, there's still more HPV cancer than there could be.

So, vaccinate your daughters *and* your sons (and your non-binary children too!)!

But HPV isn't the only infection that causes cancer.

Another example: Heliobacter pylori

This nasty little bacteria can cause stomach (gastric) cancer.

H. pylori nestles into your stomach lining.

Your stomach is full of acid that you need to break down food, but H. pylori doesn't like that, so it converts the acid to ammonia which your stomach really really doesn't like!

The damage from ammonia can cause ulcers and cancer.

We can treat H. pylori and maybe even eradicated it, but there's an interesting twist :

Because H. pylori destroys some of your stomach acid, it also helps prevent acid reflux.

And acid reflux can cause esophageal cancer.

So H. pylori *might* help prevent esophageal cancer?

Does that mean we should all go get ourselves infected with H. pylori?!

No. That would be like the old lady who swallowed a spider to catch the fly.

Instead, we can treat H. pylori and acid reflux, and prevent both kinds of cancer.

What other infections can cause cancer?

Epstein-Barr Virus (EBV), the virus that causes mono, can also cause several kinds of cancer including several types of lymphoma and nasopharyngeal cancer.

(This nasty virus has also been implicated in multiple sclerosis)

Epstein-Barr Virus comes from a family of viruses called Human herpesviruses (EBV is HHV-4).

Another member of this family, HHV-8, causes Kaposi's sarcoma which is a cancer of the skin and mucous membranes, including the lining of the stomach & intestines.

For those with a passing familiarity of the history of HIV/AIDS, you'll recognize Kaposi's sarcoma as one of the opportunistic diseases which first tipped us off to the existence of HIV.

HHV-8 thrives when our immune systems are worn down by other infections like HIV or malaria

Another type of skin cancer, called Merkel cell carcinoma, is probably caused by a virus called Merkel cell polyomavirus.

Merkel cell carcinoma shows up as a fast-growing nodule that might be skin-toned or might be red, or purple, or even blue, and that bleeds easily.

And then there are the hepatitis viruses. These viruses infect the liver and can cause liver cancer, plus sometimes other cancers.

Hepatitis B and Hepatitis C are the main cancer-causing hepatitis culprits, and can cause liver and bile duct cancer, and non-Hodgkins lymphoma.

Hepatitis D probably also causes liver cancer, and people who have both Hep B and Hep D have an extra high cancer risk.

There's also a Hepatitis E which might cause liver cancer, and a Hep A which (probably?) doesn't.

Hepatitis viruses are no joke but luckily we have vaccines and treatments for most them now.

Prevent liver disease & cancer by getting your Hep A and Hep B vaccines, and by being tested for Hep C so that you can get treatment.

If that list of viruses & bacteria isn't enough, there are also several parasites which can cause cancer.

Liver flukes are a type of flatworm. You can get infected by eating fish infected with baby worms. As adults, they like to live in your bile duct & cause bile duct cancer.

Schistosoma are another kind of flatworm--these ones are blood flukes.

The name of the acute disease they cause is schistosomiasis, or bilharzia, which is nasty enough -- chills, fever, rash, pain-- but these worms can also cause bladder cancer.

That's just some of the infections that we *know for sure* can cause cancer.

We've only just started to understand how this happens, but we know it often takes years, or even decades, for infection to lead to cancer.

There are almost certainly more out there.

What about the idea that some bacterial infections are helpful for our immune systems?

Well, for starters, that's not quite the right question. It's probably not *infection* with certain bacteria that helps prevent some allergies -- it's just inhaling some of their proteins.

Some types of bacteria have a cell wall that contains a protein called lipopolysaccharide (LPS) which (together with some other proteins) makes up something called 'endotoxin'.

When people inhale endotoxin (or even just LPS), our immune systems react (no infection needed!)

For people who grew up on/near a farm, there's *some* evidence early life, low-level exposure prevents respiratory allergies.

On the other hand, workers exposed to endotoxin in dust, particularly cotton & textile workers, have higher rates of lung cancer, plus COPD & emphysema.

But what about the claim that respiratory infections prevent cancer?

Again, wrong question. The little evidence we have is for *inhaling* endotoxin NOT infection. (FYI, viruses don't do endotoxin)

Plus our only data comes from studying workers which is notoriously hard!!

A big challenge with studying workers is people who are sick often can't work. This is called 'healthy worker bias' and is a topic for another thread.

I'm out of space on this thread, but one of my very first papers reviewed endotoxin. Read it here: bit.ly/3rmcBkc

Also, shout out to @jenniferrrider.

Her Infections & Cancer class, which I took during my doctorate @HarvardEpi, helped me put this thread together.