Leucogen (feline leukaemia vaccine (inactivated)) is used in cats to protect them against feline leukaemia, an infectious disease of cats which affects the immune system and is caused by the feline leukaemia virus (FeLV). Signs of the disease can include loss of appetite, weight loss, poor fur condition, fever, pale gums and diarrhoea; cats persistently infected with the virus may spread it to other cats.