Many chronic diseases and cancers are associated with infectious microbes. By this it is meant that the associated microbe is found in the tissues of patients with the disease more frequently than in the bodies of healthy people. When a microbe such as a virus, bacterium, fungus, protozoan or archaeon has been linked to a disease, researchers may start to investigate whether that microbe might be the underlying cause of the disease, or might play a causal role in the illness in conjunction with other causal factors.