Feline leukemia virus is a retrovirus that infects cats. FeLV can be transmitted from infected cats when the transfer of saliva or nasal secretions is involved.
Feline leukemia virus (FeLV) is one of the most common infectious causes of disease of cats globally. Infection with FeLV can cause a variety of clinical signs, impacting a cat's longevity and quality of life.
ABCD cats & vets: GUIDELINE for Feline Leukaemia Virus Infection.

Common symptoms of Cat Leukaemia: Lethargy: Cats infected with FeLV often exhibit increased fatigue and a lack of interest in activities they once enjoyed. Weight Loss: Unexplained weight loss can be a sign of various health issues, including FeLV.
What Causes a Lethargic Cat? Lethargy in cats is quite a vague sign, with many potential causes, some more serious than others.FeLV (Feline Leukemia Virus) or FIV (Feline Immunodeficiency Virus). Feline Infectious Anemia (caused by a blood parasite, mycoplasma haemofelis).

Cat Diseases and Symptoms. Cats can develop a variety of health problems throughout their lives.Lethargy and depression. Fever that does not respond to antibiotics. Fluid accumulation in the abdomen or chest, causing swelling and difficulty breathing.
Learn how to recognise lethargy in cats, understand common causes and when to seek vet care, plus practical tips to help your cat feel better.Infections: Feline immunodeficiency virus, Feline leukaemia virus, Lyme disease, Leishmania.