Rob, December through to April is the peak Malaria season in Kruger (rainy season). Most lodges and safari tour companies take good precautions against mosquito bite (bed nets, repellent coils and spray etc) and the incidence of tourists getting Malaria is quite low.
Best if you talk to your doctor about anti-malarials, some people get quite nasty side-effects like hallucinations and insomnia, and because the treatment is the same as the prevention, some people wait until they're infected before taking them. But again, that's a decision for you after getting proper medical advice from a doctor not a journalist (me).
On a personal note, I was in Kruger during the dry season a few years back and decided not to take anti-malaria medication, but I used a lot of insect repellant on my skin. (btw, it was a fabulous experience.)
Phil from the safety hub. almost 7 years ago