But bacteria are not all nasty. Your intestines are crammed with E. coli bacteria, which help you digest your food, and bacteria help in many ecological processes. More on disease bacteria . Like viruses and bacteria, parasites live at the expense of other organisms. However, a membrane around the cell nucleus indicates that they are more evolutionarily developed than bacteria.
Parasites can have one or many cells.
Single-celled parasites include malaria, the deadliest infection on Earth, which kills two to three million people each year. Like many viruses, the parasite is transmitted by mosquito bites. Other single-celled parasites include Giardia and amebic dysentery, both found in foul drinking water, and both causing severe diarrhea. Multi-cellular parasites include trichinosis, hookworm, roundworm and tapeworm. Often found in soil, wood or decaying vegetation, fungi are actually closer in evolutionary terms to us than to bacteria.