I predict its not some sort of mosquito-borne illness, but the mosquitoes are actually monsters and they are sucking their mana or vitality or something like that
I don't know about stats but to me, I find dengue to be less worse of the two. I caught dengue once and the doctors only prescribed me rest and water (and the usual common pills). It was a terrible week but I'm okay now.
You're right malaria is worse than dengue since it has a higher fatality rate especially to him since he's probably malnourished af and malaria causes organ failure, and dengue can develop into severe dangue and cause internal hemorrhaging if left untreated but has a significantly lower mortality rate than malaria.
The mosquitoes seemed to bite the girls more, probably they suck mana. Dai will be unconscious for a time, and the girls will need to cooperate to survive by themselves without any of their powers because of the lack of mana.
Not that I expect realism from this manga but, when it comes to Malaria, it usually takes around 10-15 days to show symptoms after the mosquito bite (5-10 days for Dengue). Exhaustion and malnutrition might "help", but these infections do not work this fast (nor cause people to lose consciousness). For now, I'll file it under "magic illness".
Indeed also for those who aren't inthe know.. 'severe malaria' has a nearly 100% fatality rate if untreated so yea, author might need to bust out some magical healing if they're going this route