Most microorganisms are innocuous but some can be extremely pathogenic. Whether a person gets a contagious or transmissible disease depends on the type and number of the infectious agents the person interacts with.
| Type of infection | Pathogen | Transmission agent |
| Gastrointestinal infections | Cholera, salmonella | Fecal matter, vomit |
| Respiratory infections | Pneumococcal disease, tuberculosis | Inhaled droplets from infected individuals' exhalation |
| Eye infections | Herpes | |
| Skin infections | Bacterial skin disease | Fecal matter, vomit |
| Meningitis | Meningitis bacteria | Cerebro-spinal fluid |
| Haemorrhagic fever | Ebola | Blood and secretions |
| Avian influenze | H5N1 virus | Blood, feces |
| AIDS | Human Immunodeficiency Virus (HIV) | Bodily fluids |
| Hepatitis | Hepatitis virus | Bodily fluids, feces |
