Its important to remember that human bodies are effectively an environment unto themselves, and that there really isnt a distinct “you” and then other things like this.

For example, there are trillions upon trillions more cells of bacteria on/in you than there are human cells. Every person’s overall existence is composed primarily of stuff that isnt technically “you”, although it is inseparable from you. While people’s bodily ecology can be modified, you couldnt possibly remove every quasi-foreign substance from yourself and still survive.

Its part of why seemingly horrific stuff like fecal transplants can be very helpful. If youve cultivated a very mean ecology for one reason or another then resetting that to a healthier state is essential. Its like the gut version of you being a country suffering from unrest. But even though we think of negative or positive bacteria as being totally distinct from us, we are strongly defined by those bacteria. It affects your brain and emotions far more than most people realize. Scientists are still just beginning to understand the complexities of these systems, too.

These little dudes I would assume are a similar situation. They probably have some form of essential symbiosis with us that we may not even understand yet. Killing them off would likely be bad. And even if you did, you would be creating a vacuum that could be filled by something worse. Our bodies are fertile ground for such tiny individuals. Better to have benign ones or helpful ones than it is to have bad ones