Traditionally, cancer has not been considered an infectious disease although some multiple cases of leukemia in man and cattle have been reported. The discovery that feline lymphosarcoma was associated with an RNA virus (feline leukemia virus(FeLV)) meant that infectious transmission of the disease was a possibility. The critical question was whether the predominant method of transmission from one animal to another was 'vertical' (via the gametes) or 'horizontal' (via contagion or infection). A number of epidemiological studies have shown that the chances of healthy cats contracting lymphosarcoma are greatly increased when a cat with the disease lives in close proximity. It does not matter whether the healthy cats are related to the sick animal or not. It has also been established that viremic normal cats have an approximately 900 times greater chance of developing leukemia than cats whose FeLV status is unknown. Infectious FeLV is present in the excretions and blood of viremic animals. In the natural environment, feline lymphosarcoma occurs in clusters. The results in pet cats have been supported by experiments with cat colonies under controlled conditions and prove that horizontal transmission of FeLV occurs. This does not mean that epigenetic (infection in utero or via the milk) or vertical transmission cannot also occur. It should be possible to break the cycle of horizontal transmission of the virus by vaccination and thus control FeLV-related diseases.