One would think that whole blood would be a very common element in a parasite's diet. And indeed for many of the larger ones it is. However, for many of the smaller parasites, the plasma is all they can get to, and this lacks many of the essential nutrients a cell needs. It is partially for this reason that the blood is not the most popular place for a parasite to eat. Mostly larger parasites may eat whole blood, and even they tend to eat other cells as well to supplement their diet.

Another problem is immune response. The body both walls off and attacks foreign bodies. In order to survive an organism must address this problem as well.

Finally, while the blood is less toxic than, for example, the intestine, it is more difficult to get to. One needs a specialized method of delivery, rahter than simple ingestion, in order to live here.