(deleted my last post after reading more on the subject) All forms of partial colorblindness can distinguish red and yellow to some degree, so your color choice is fine. Making the red more saturated would help even with total colorblindness, as would the symbol.

a little plus or some other small mark on the icon would also help to differentiate them

Flip/Mirror one of the mosquitos around ?

or add a small skull above the severe malaria mosquito ?

I like the idea of mirroring the misquito on one, and adding an '!'

I think that normal malaria should be just one mosquito while severe malaria can have multiple (like 3) mosquitoes.