[Babul]

theres an invisible object somewhere, spawn it as pickup at the fires location, and check for any player touching it. if so, decrease his health by 5 works, but the pickup code gets called not often enough, so a timer would make this a bit more precise (lose 0.1 HP each 100 ms for like 2 seconds, instead of losing 10 HP each time the pickup "triggers" after a few seconds)...