These are viruses that use the cat's genes to reproduce. Until the 1970s these viruses were virtually unknown. FeLV is further classed as an oncornavirus or cancer-causing virus. The cancers it causes are leukemia and lymphosarcoma, but it can also have many other effects on the host body. It commonly suppresses the immune system and allows what would not be a disease-causing organism to cause illness.
Transmission is by direct contact.
This means that the virus has to go from an infected cat's secretions directly to another cat. More importantly, the virus must get into the blood stream quickly. This means that a cat who never ever goes outdoors and never ever comes in contact with another cat has virtually no chance of being infected. Usually fighting and grooming are the main ways the disease is passed. Kittens can be infected during and after birth also.