Which virus is known to be fatal in cats, leading to decreased white blood cells and impacting the immune system?

The Feline Leukemia Virus (FeLV) is a viral infection that significantly impacts a cat's immune system by causing a reduction in the number of white blood cells, which are critical for fighting infections. This virus can lead to various health issues, including anemia, lymphosarcoma, and susceptibility to other infections due to its immunosuppressive effects. Cats that are infected with FeLV often experience a poor quality of life and can have a decreased life expectancy, as the virus can be fatal in many cases.

While other viruses listed, such as Feline Viral Rhinotracheitis or Feline Infectious Peritonitis, also affect cats and can be serious, they do not typically have the same profound impact on white blood cell counts or result in severe immunosuppression as seen with Feline Leukemia Virus. Feline Panleukopenia is another severe viral disease that can also lead to a decrease in white blood cells, but it is distinct from the impact of FeLV on the immune system. Thus, the specificity of FeLV in causing fatal outcomes through its effects on white blood cell levels makes it the correct choice.