That'll be Ophiocordyceps unilateralis. It invades the ant's brain and causes it to leave it's nest and go somewhere better for the fungus then wait to die as the fungus errupts from its head.
If we're talking about nightmare mind control horrors, we shouldn't forget our old friend toxiplasmosis gondii, which infects rodents, then alters their behaviour so they're not afraid of cats, in particular. This leads to the rodent getting eaten so the parasite can infect the cat, which is the only place it can reproduce, before spreading from the cat faeces back into the rodent population. It can also infect humans where there is evidence that it affects behavior too, particularly making males more careless of rules.
Oh come now, there are so many more interesting parasites that mess with your brain for their own benefit; Trypanosoma which messes with your sleep before slowly killing you, Naegleria fowleri that just straight up eats your brain and a host of others that do weird and wonderful things.
Look at it this way; before you were surrounded by mind controlled ants, suicidal rodents and other such horrors without even knowing it. Now you do know about them.
What's that? I'm really not helping? Ok, I'll stop.
Ah rabies, a delightful little horror that makes it so painful to swallow that victims will flinch at the mere sight of water, then drives them into a frenzied rage in an attempt to spread through bite wounds. It looks like they've developed a couple of protocols (Milwaukee and Recife Protocols) that give the victim a chance, even if mot a good one. They both involve an induced coma so that you don't attack anyone, so that's fun.
Get your vaccinations folks, running around foaming at the mouth and attacking anyone near you isn't a good way to go out.
I don't think they understand 'rules', rather they mess with the brain structures that control self regulation. It's believed that around 30-50% of the human population may have a T. gondii infection, with a corresponding link to other diseases. High levels may also go some way to explaining the prevalence of high risk behaviors in certain areas, although proving a correlation is challenging due to confounding factors.
The snail has been taken over by parasitic flatworms that control it to seek out exposed spots and pulsate inside of their eye stalks to get eaten by a bird and enter the next stage of their life cycle, which they spend by living in the birds cloaca and spreading their eggs via feces.