Quickie: Dragonslayer Fever

Toxoplasma Gondii is a parasitic organism that can only reproduce in the guts of cats. When cats poop, it spreads to rats (when they eat the poop, gross, I know), where it then proceeds to affect their mental state, preventing them from feeling fear at the presence of cats. The rats get eaten, then the cat has the parasite in its stomach, and the cycle starts all over again.

Dragonslayer Fever is a disease caused by a similar parasite, except this one needs, as the name might imply, humanoids and dragons. The most common means of infection is by handling gold from a dragon's horde, which can carry such parasites for several months if it is not properly decontaminated. 

It is a very dangerous infection, often highly lethal. The main symptom is courage. Dragons are (super)naturally terrifying beings, but those who have contracted dragonslayer fever meet them with unshakeable resolve. Combine that with a highly honed urge to seek out dragons and to do violence to them and you have a perfectly honed recipe for dying a glorious death. If one is infected for long enough, the disease will erode their fear response to the point that basically nothing phases the sickened individual, much to their detriment. The disease is often misinterpreted as heroism or spectacular bravery, and indeed many a village hero has secretly been harboring such a parasite, driving them to take on greater and stupider challenges as their risk assessment atrophies to the point of uselessness.  

In mechanical terms, they get advantage vs fear effects caused by dragons after the first week, then advantage vs all fear effects after two. After two more weeks, they are completely immune to fear. Every week after the second, their wisdom score (or system equivalent) decreases by 1, down to a reasonable lower limit (they're not complete idiots, it would probably cap off around leaving them with a modifier -2 or 3. The disease won't kill them by reducing the stat to 0 if that's something your system cares about.) Their wisdom decreasing also makes the infected individual worse at saves against charm in my system of choice (5e, lol) which feels right somehow.

I've heard that toxoplasma gondii causes rats to associate the smell of cat urine with sex, so maybe that's what's going on here, the idea of killing dragons is getting your fighter hot and bothered, to the point that his mind is so distracted he can't put up proper defenses against the charm effects of the green dragon they're fighting. That's why it makes clerics worse at spellcasting too, thinking too many impure thoughts, your god does not approve of such lasciviousness.