Viruses are unique noncellular pathogens which are composed of segments of DNA or RNA and a capsid (protein envelope). They must grow and replicate within the living cells. After viral infections, more progeny viruses are produced rapidly and destroy the host cells, resulting in diseases.
Bacteria are a group of one-cell organisms. They are usually larger in size than viruses and appear in shape as rods, spirals, or spheres under the microscope. Bacteria are responsible for a variety of disease ranging from asymptomatic to sudden and intense, such as strep throat, meningitis, pneumonia, and hemorrhagic colitis. However, not all bacteria have the potential to cause diseases and many bacteria are beneficial for human health, such as intestinal flora.
Parasites are tiny organisms that live in or on a host and get food or energy from that host. There are three types of parasites causing human diseases, including protozoa, helminths, and ectoparasites. Protozoa are single-celled organisms. Helminths, also commonly known as worms, are larger, multi-celled organisms. Ectoparasites are multi-celled organisms that live on or feed off human skin.
