- Viruses are the smallest. It's structure consists of a protein outside coating. It causes illness by hijacking the cell to produce more virus, and then eventually destroying the cell. It is not affected by antibiotics
- Bacterium are the medium sized. Their structure consists of a cell with no nucleus. It causes illness by releasing toxins into the body, using food to multiply. It is effected by antibiotics.
- Fungus are the largest of the pathogens. They are either singular or multicellular, with hyphae, which are long strands. They cause illness by releasing enzymes which it uses as food. Not effected by antibiotics.