Over the past century, the incidence of cancer rose markedly in developed countries. Many underlying factors are known, but not all of them. Over the same century, advances in public health and hygiene practically eradicated many infectious illnesses, and exposure to some untargeted infectious agents has also declined. There are reasons to suspect that these two trends are linked. Although some viruses and one bacterium are known to be oncogenic, a high percentage of infectious agents, 71% of viruses tested in one study, have shown oncolytic properties. Some of these agents have minimal or no clinical consequences, and some are - or were - quite ubiquitous. Many types of cancer are unusually susceptible to several different agents, sometimes 10-1000 times more susceptible than normal cells. When wild-type or attenuated viruses are used to treat cancer, they are often more effective with smaller tumors, and it may be that nascent, undetectable cancers are especially vulnerable. Environmental exposure to bacteria and viruses may provide natural protection from cancer by attacking it at this stage, or inducing the body to do so. Immunity to an infectious agent can attenuate its oncolytic effect, but generally does not stop it, so both initial and repeated exposures may be protective. As more direct evidence, treatment of animals with selected live virus vaccines has provided protection from cancer, and in humans, exposure to two specific infectious agents is known to correlate with a reduced risk of cancer. The significant decline in exposure to infectious agents over the past century may have inadvertently weakened this naturally protective mechanism, driving cancer rates up. There is considerable research on the impact of oncogenic agents, but surprisingly little regarding the impact of oncolytic agents on cancer rates. An understanding of the relationship between natural infections and the suppression of cancer may lead to prophylactic measures against cancer, improved public health policies regarding vaccination programs and hygiene, and greater insight into cancer treatment as well.
Bacterial Infections Environmental Exposure