Some recent experimental work has suggested queens may be able to pass infection on to their offspring, possibly via suckling. FeLV and FIV can be found together in domestic cats that are infected. However, there is no requirement for co-infection. Both suppress the immune system, so infection with one might allow easier infection with the other. Other studies and other countries have shown FIV rates from single digits to around a quarter of sampled cats.
Immunosuppression is a very complex topic.
The clinical course of disease in cats closely approximates HIV infection in people. The end result of immunosuppression is that other infectious diseases are allowed access to the body, and death is usually the result of the other infections. In veterinary medicine, death is often from euthanasia. The disease has been described in stages, similar to those used for HIV infections in people. There is an initial infection which may show diarrhea, fever and a depressed state.