Ectoparasites, such as fleas, can spread the virus when they feed from multiple hosts. Panleukopenia virus is an extremely hardy virus and survives most temperatures and disinfectants. It is possible for a cat to get infected both directly and indirectly at the same time, but the hardiness of the virus makes both ways equally infective. The cat who is shedding virus could have done so a long time before the next cat comes across it and becomes infected as well. Also, common environmental contamination both indoors and out can be assured due to the resistance of the virus to all disinfectants BUT bleach.
Strict isolation is essential to reduce the environmental contamination.
Veterinarians will often prescribe antibiotics as well, but only because they wish to avoid the resulting bacterial infections that happen in conjunction with the damage from the virus infection. Supportive care is dependent upon which symptoms and which form of the disease was produced. Whole blood transfusions can be used to help replace WBCs. There is no treatment for this form of the disease. By the time it becomes evident, the damage is done.