We usually think of infections as short-term problems, like fevers, coughs, maybe a stomach bug.
But some infections don’t just go away. They linger.
And when they do, they can leave a much bigger mark.
What kind of mark? Cancer.
Yes, you read that right.
About 1 in 6 cancers worldwide are actually caused by infections.
I've been fascinated by this intersection of infectious diseases and cancer throughout my career,
and thanks to powerful new tools like metagenomics (more on that in a second), we’re discovering even more surprising links between the microbes we carry and the diseases we fear.
Let me walk you through this incredible story of dangerous connections and why it gives me so much hope for the future of cancer prevention.
When most people think about what causes cancer, they picture cigarettes, toxic chemicals, or bad genes passed down through families.
But there's a whole other category of cancer triggers that's been hiding in plain sight: infectious agents.
The World Health Organization has officially recognized ten infectious pathogens that have been classified as carcinogenic to humans (Group 1) by the International Agency for Research on Cancer (IARC)
- One bacterium, Six viruses and three parasites.
The Viruses
Human Papillomavirus (HPV) - You've probably heard of this one. It causes cervical cancer, but also throat and anal cancers.
Hepatitis B and C viruses - These liver infections can lead to liver cancer years later.
Epstein-Barr Virus (EBV) - The same virus that causes "mono" in teenagers can also trigger certain lymphomas and stomach cancer.
Human T-lymphotropic Virus (HTLV-1) - Causes a rare but aggressive blood cancer.
Human Herpes Virus 8 (HHV-8) → causes Kaposi’s sarcoma, mostly in people with weakened immune systems.
HIV, though not officially on the WHO list, has been associated with various cancers because it weakens the immune system, making other cancer-causing infections more dangerous.
The bacterium
Helicobacter pylori (H. pylori) - This stomach bug causes ulcers, it can also lead to stomach cancer
Streptococcus gallolyticus is an interesting one: when we find this bacteria in someone's blood, we immediately check for colon cancer because up to 65% of people with this infection also have colon tumors.
The parasites
Liver fluke infections from Opisthorchis viverrini or Clonorchis sinensis can progress to liver cancer and/or gall bladder cancer.
Blood fluke infections with Schistosoma haematobium can progress to bladder cancer.
What About the Microbes We Haven’t Found Yet?
This is where metagenomics comes in.
Instead of testing for just one or two bugs, metagenomic sequencing scans for everything - bacteria, viruses, fungi, and even parasites - in a single test.
It doesn’t rely on us knowing what to look for. It just… finds it.
Thanks to this tech, we’ve started spotting unusual bacteria in tumors that weren’t on anyone’s radar
For example, Fusobacterium has been found in colon cancer tissue and it might actually be helping the cancer grow and resist treatment.
That’s the power of metagenomics: it opens our eyes to what’s really going on behind the scenes.
..to be continued
Stick with me! next week, we’ll explore even more about how these sneaky pathogens trigger cancer, how metagenomics is helping us connect the dots, and how we can turn that knowledge into prevention.