Pathogens

There are three categories of bacterial pathogens. Obligate pathogens are those bacteria that must cause disease in order to be transmitted from one host to another. These bacteria must also infect a host in order to survive, in contrast to other bacteria that are capable of survival outside of a host. Examples of obligate bacterial pathogens include Mycobacterium tuberculosis and Treponema pallidum.

Opportunistic pathogens can be transmitted from one host to another without having to cause disease. However, in a host whose immune system is not functioning properly, the bacteria can cause an infection that leads to a disease. In those cases, the disease can help the bacteria spread to another host. Examples of opportunistic bacterial pathogens include Vibrio cholerae and Pseudomonas aeruginosa.

Finally, some bacterial pathogens cause disease only accidentally. Indeed, the disease actually limits the spread of the bacteria to another host. Examples of these "accidental" pathogens include Neisseria meningitides and Bacteroides fragilis.