Infectious diseases can be caused by many pathogens, including bacteria, viruses, fungi, and parasites that may cause illness and disease. For humans, transmission of pathogens may occur in a variety of ways: spread from person-to-person by direct contact, water or foodborne illness or aerosolization of infected particles in the environment and through insects (mosquitoes) and ticks.
Infectious diseases in humans are caused by microorganisms including:
- Viruses that invade and multiply inside healthy cells
- Bacteria, or small, single-celled organisms capable of causing disease
- Fungi, which include many different kinds of fungus
- Parasites, which are organisms that live inside host bodies causing sickness