Malaria parasites are capable of both asexual and sexual reproduction.<p>While the only reproduce asexually within the secondary host the reproduce sexually within the primary host (mosquitoes) this means that if you have parasites that individually develop a resistance to different drugs within the secondary host (humans) could result in combined resistance once they sexually mate back within the primary host.<p>Malaria is also one of the most polymorphic single cell organisms in existence which is one of the main reasons why vaccination attempts were pretty futile so far which might give it a much better chance to develop resistance.