Pathogens are microorganisms that cause disease. The study of pathogens is called microbiology. Pathogens can be bacteria, viruses, fungi, or protozoa. They can be spread through the air, water, food, or contact with contaminated surfaces. Some pathogens only cause disease in certain animals or humans. Others can cause disease in any living thing.
Most diseases are caused by more than one pathogen. For example, the flu is caused by a virus, but it can also be made worse by bacteria. Pathogens often work together to cause disease. This is called synergism.
Some pathogens live inside other organisms (such as parasites). These are called obligate intracellular parasites. Other pathogens only live and grow outside of cells (such as viruses). These are called extracellular parasites. Most bacteria are free-living and not parasitic; however, there are some pathogenic bacteria (such as those that cause tuberculosis and leprosy).
There are four main types of infection: viral, bacterial, fungal, and protozoal infections. Viruses infect cells and use them to reproduce; they cannot reproduce on their own. Bacteria also infect cells; however, they can reproduce on their own outside of cells (in soil or water for example). Fungi generally live on dead organic matter; however some fungi (such as yeasts) can infect humans and other animals causing infections such as thrush or athlete’s foot . Protozoa are single-celled organisms that usually live in water; some species of protozoa can cause disease in humans (such as malaria).