how to make mob deal custom damage type

im trying to make a infection / parasite mod for minecraft, and cant figure out how to get the infector mob to turn spiders into their infected counterpart, the image i followed this https://postimg.cc/DJBYfCgg and it says " is provided damage source of type [ parasite ] but i cant find out how to make my mob deal parasite damage, ive made the damage type, the infector, and most of the image, i just need to figure out how to make the infector mob deal parasite damage :[