Viruses: Viruses are tiny infectious agents that can only replicate inside living cells. They hijack the host cell's machinery to reproduce and spread.
Bacteria: Bacteria are single-celled microorganisms that can be found almost everywhere. While some bacteria are harmless or even beneficial, others can cause infections and diseases.
Parasites: Parasites are organisms that live on or inside another organism (the host) and rely on it for survival. They harm the host by taking nutrients or causing damage to tissues.