1. Bacteria: These are single-celled microorganisms that can cause a wide range of illnesses, from food poisoning to pneumonia. Examples include E. coli, Salmonella, and Streptococcus.
2. Viruses: These are even smaller than bacteria and consist of genetic material (DNA or RNA) surrounded by a protein coat. Viruses can only reproduce by invading and taking over living cells. Examples include the influenza virus, HIV, and the COVID-19 virus.
3. Fungi: These are eukaryotic organisms that can be single-celled (yeasts) or multicellular (molds). Some fungi cause infections like athlete's foot and ringworm, while others can be deadly, such as those causing cryptococcosis.
4. Parasites: These are organisms that live on or inside another organism (called a host) and benefit at the host's expense. Some parasites are microscopic, like protozoa (e.g., malaria parasite), while others are larger, like worms (e.g., tapeworms).
