A pathogen is an infectious agent; colloquially, a germ. The types of pathogens include prions, viruses, bacteria, fungi, and other parasites. Archaea, from which eukaryotic life arose, are the only microbial life form which is entirely benign.
Prions are misfolded proteins that propagate by transmitting their mangled state to other proteins. A prion acts as a template, inducing properly folded proteins to take the prion configuration.
One way that prions are transmitted to humans is by eating infected meat: cattle with mad cow disease (bovine spongiform encephalopathy), which causes Creutzfeldt–Jakob disease in humans.
Some fungi have proteins showing prion-like behavior, but fungal prions do not appear to harm their fungal hosts.
Viruses are stunningly cunning packages that hijack cells to reproduce. Viruses invade all organisms, including other viruses.
Viruses are major players in horizontal gene transfer, which greatly improves their adaptability. While this has had a major beneficial impact on evolution, it offers no consolation to the infected.
Bacteria comprise a huge domain of single cell prokaryotes, coming in a wide range of shapes and lifestyles. Bacteria are ubiquitous in every livable habitat on Earth, including some unlivable habitats where few other species care to dwell. Bacteria have cozied up to every multicellular life form, often symbiotically, though sometimes the occupation is less than cozy for the host.
The most common fatal bacterial diseases are respiratory infections. Tuberculosis is an airborne bacterial disease. The bacteria itself does not wreak havoc. Instead, a body’s immune system damages the lungs in an incessant war to purge the infection. 1/3rd of the world’s population may be infected with tuberculosis.
Human parasites include helminths (parasitic worms) and protists. Humans acquire helminths from contaminated water, fish, and meat. Mosquitoes are helminth delivery agents. However they enter the body – mouth, anus, nose or skin – most helminth species attach themselves to the intestinal tract.
Protists comprise a vast variety of unicellular eukaryotes that live anywhere that has water. When living as a colony, protists do not differentiate into tissues. Many protists, such as algae, are photosynthetic. Marine plankton are protists. Protists are typically vital primary producers in an ecosystem: the bottom of the food chain. The most notorious pathogenic protist delivers malaria.