FeLV is one of the most frequent feline viruses causing an incurable disease known as cat leukaemia. It is a retrovirus and infects cats only: it does not pose a risk to humans or other animals (apart from felids). The FeLV is excreted by a sick cat with saliva, faeces, urine and milk. It is sensitive to a high temperature, sunshine and detergents. It can live outside the host’s body for several minutes. Therefore, the infection requires a direct contact of a healthy cat and a sick cat. The shared use of a bowl, mutual grooming (licking) or bites are the most common infection sources. The most common symptoms of the disease include apathy, loss of appetite and weight loss which result from anaemia. With the progress of the disease, the cat becomes more susceptible to recurring respiratory and alimentary infections. The first symptoms of the disease can occur even a few days after infection; thus, it is very useful to examine the cat (particularly, the cats leaving a house or remaining in a large clowder) to avoid the transmission to other cats. Isolation and vaccination effectively prevent the disease from spreading.