1. Bacteria: These are single-celled organisms that can cause a wide range of infections, from pneumonia to food poisoning. Examples include *Streptococcus pneumoniae* (causes pneumonia) and *Salmonella enterica* (causes food poisoning).

2. Viruses: These are even smaller than bacteria and consist of genetic material encased in a protein coat. Viruses can only reproduce inside living cells. Examples include the influenza virus (causes the flu) and the HIV virus (causes AIDS).

3. Fungi: These are eukaryotic organisms that can be single-celled or multicellular. Some fungi are beneficial, such as mushrooms, but others can cause infections. Examples include *Candida albicans* (causes yeast infections) and *Aspergillus fumigatus* (causes lung infections).