Answer: Plasmodium falciparum malaria.
This case has classic features of P. falciparum infection, including multiply-infected RBCs, small delicate ring forms in RBCs that are not enlarged, and numerous applique forms. The parasitemia is also quite high. As anonymous mentions, "it looks like 20%, which is in the 'ack!' range." The thick smear in this case is especially impressive!
Fortunately, this patient was promptly treated and did well on medication alone. At last count, her parasitemia levels were undetectable. She did not undergo exchange transfusion, but this certainly was a consideration, given her high parasitemia.