FeLV and FIV both can cause the suppression of the immune system.
They are both viruses and therefore cannot be cured with medications.
FIV is a lot like HIV: It can lie dormant a long time before it starts killing the white blood cells. It can be passed sexually, but unlike HIV it is most commonly spread through bite wounds.
FeLV can suppress the immune system, but it gets its name from the fact that it can cause bone leukemia and cancers elsewhere in the body because of how it reproduces.
Young cats are more easily affected.
There is not an effective FIV vaccine, but there is an effective FeLV vaccine.
Test first, then vaccinate and keep your cats away from infected individuals.