Infestare means to be over-run in latin.
According to my parasitology lecturer infestation is used for the
particular parasites that live in and around the skin. So external
parasites give an infestation and internal parasites an infection. So
in your case I would say if its in the blood the parasite probably is
in an infection.
BSc. in Microbiology and Development Sociology
Cornell University/University of Melbourne (Australia)