There are cases wherein a virus is lying latent within an animal or plant. There is enough evidence to prove the existence of such latent viruses. Viruses can be examined under a high-powered microscope using high power microscopy. While they are not concerned with tumor-formation, nevertheless their very existence shows that there may be other latent viruses not yet recognized which are concerned in the production of tumors. With regards to latent viruses in animals, the disease of Herpes simplex or “cold sore” in man seems to be a case in point. It is common knowledge that herpes develops more or less spontaneously. The stimuli which cause it to develop vary from fevers to emotional disturbances. Herpes is known to be caused by a virus, and the most widely accepted explanation is the presence in the body of latent herpes virus which can be stimulated into action by one or other of the causes mentioned. It is thought that the herpes virus enters the body in early childhood and persists throughout life in certain susceptible individuals.

Traub experimented on the virus disease in mice known as Lymphocytic choriomeningitis. Traub described this virus, which affects both mice and men, as an extraordinary example of how equilibrium between host and virus can be set up. Traub discovered that the virus persisted in the blood of mice after recovery from infection and that female mice thus carrying virus could infect their young in utero that is before birth. When infected like this, the young mice showed symptoms of disease, but other young mice infected after birth by contact showed no definite symptoms. It was discovered using high power microscopy that it was possible to provoke symptoms of the disease, however, by injection with sterile broth, a proceeding which seemed to upset the host-virus balance. After about two years of continued breeding of these mice the situation seemed to change. The disease became milder till it produced no symptoms at all. Moreover, infection of young mice in utero became the invariable rule apparently because all stock mice, young and old, were now virus carriers. The presence of virus in them could only be demonstrated by inoculation to another strain of mice which carried no virus and were not immune and observation under a microscope.

The use of mice in such large quantities as experimental animals has revealed the existence of other hidden viruses which come to light and produce symptoms only after a long series of passages through apparently normal mice. The reason for the virus becoming apparent after such serial passages is not clear, unless it be that such a treatment being favorable to the virus enables it to build up into so concentrated a form that a disease results.

Another interesting disease of fowls which is known as fowl paralysis is almost certainly due to a virus. The paralysis is due to the formation in the nerves of tumor-like growths. Some evidence obtained using high power microscopy suggests that fowl paralysis is a stage in a disease which may be largely symptomless. It is thought also that the agent may be transmitted through the egg and even passed on through the male parent. There is additional evidence which shows that environmental conditions may determine whether the latent infection will develop into the full disease. Blakemore describes how chicks from an infected stock were divided into two lots, and how those reared at one farm developed symptoms and those kept at another farm did not.

Latent viruses occur also in plants, especially in potatoes, strawberries and hops. In the United States of America potato plants are so universally affected with a particular virus which produces no symptoms. For this reason it has been named the “latent or healthy potato virus.” Changes in environ¬mental conditions can also upset the host-virus equilibrium in plants. Symptoms of the disease are frequently masked by both high and low temperatures.

Plenty of evidences resulting from high power microscopy show that viruses can lie latent in both animals and plants. There is also definite evidence derived from high power microscopy that there exists some connection between the tar and viruses. For instance, there is a virus disease of rabbits known as Shope’s infectious fibroma. This virus disease produces tumor-like outgrowths which last for a few weeks and then disappear. If, how¬ever, tar is injected beneath the skin of the rabbit, the regression of the tumors is delayed and they frequently develop into invading growths which cause the death.