The feline leukemia virus (FeLV) is the causative agent of the most important fatal infectious disease complex of domestic cats. FeLV is transmitted mostly by direct contact via saliva, nasal secretions, and milk by mutual grooming or occasionally by bites. It can also be transmitted indirectly via contact with faeces from FeLV-viremic shedders. Transmission from mother to offspring also occurs occasionally. Young kittens are especially susceptible to FeLV infection while with increasing age cats become more resistant to infection. The majority of in-practice tests for FeLV detect viral antigen (capsid protein p27) in blood, plasma or serum. P27 is the most abundant viral protein in the plasma of viraemic cats as a diagnostic marker. FeLV is the most common cause of cancer in cats, may cause various blood disorders, and may lead to a state of immune deficiency that hinders a cat's ability to protect itself against other infections. After being infected with FeLV, common bacteria, viruses, protozoa, and fungi all can cause severe illness in cats.