Answer:
In general, sexual reproduction allows genes of two different creatures to recombine with each other. Basically, different combinations of genes might be more effective at dealing with parasites. Actually, this combining of genes can help facilitate gene mutation, and is the primary way that species evolve. Pressures such as parasites are just the type of thing that give species reason to evolve: some individuals are better at surviving, and thus they are more likely to spread their genes to the next generation, which results in a next generation better able to deal with the pressure.