Malaria is caused by single-celled parasites that are carried by mosquito. When an infected mosquito bites a person, some of the parasites get into the blood, feed inside red blood cells and attack the liver.
Rotifers are related to roundworms (nematodes), even though they look nothing like worms. There are 2000 different species, mostly living in ponds or on wet mosses in woodlands.
Heliozoans have spiky skeletons. They live mainly in ponds and lakes. The spikes are made of silica, the same substance that makes glass.
Daphnia are related to crabs and shrimps. They can just be seen with the naked eye, but you need a microscope to see their structure. They are transparent, so you can watch their stomach digesting food and all the other internal organs. They swim by beating their brush-like antennae.