This is your brain. This is your brain on drugs.
Anyone with a memory of the 1980s should have come across this advert somewhere, but little did they know that within their very own brain, it was likely that a parasite was already living.
Toxoplasma Gondii is a species of parasitic protozoa that lives in cats and other warm-blooded animals and can cause the disease toxoplasmosis in humans. Specifically it lives in the brain and slowly "feasts on the goo inside". Largely passed onto humans via eating meat, because its natural host is the common housecat, people who own cats are more likely to have it inside their own brain.
It is estimated that 50% of the world is infected, but this varies from country to country with 22% in the UK, 87% for France, 16% for the USA and 79% for Australia. This is really scary that half of the world have a parasite in their brain, and you know what, there's literally no known cure or way to fight this thing.
Toxoplasmae are hardy little bastards (I deliberately use this word for all implications) and have a livable temperature range of -5°C to to 65°C. Human body temperature is 37.4°C which is bang in the middle. They can hibernate for up to 12 years which is kind of scary and on top of all this, they're virtually resistant to all known chemicals.
In fact the only known cure is to remove the host... you can guess the implications of that. Worse is that it's assumed that they're at least a contributory factors in things like Alzheimer's and Schizophrenia.
So what can you do about it? Pretty well much nothing.
This is your brain. This is your brain on drugs. This is your brain being eaten alive... urk.