"A cat can appear healthy one day and be sick the next," he said.
The shelter currently has 64 cats, Hardy said. In the first six months of this year, the shelter took in more than 800 cats and kittens. In the same period, 217 were adopted.
Twelve cats died from the disease in the shelter, he said. The last one died Wednesday.
There is an effective vaccination for the disease, but most cats that come into the shelter have not been immunized, he said.
Hardy thinks the disease got into the shelter through a litter of feral kittens that was dropped off there.
"They were not vaccinated," he said.
"There's an overpopulation of what I call homeless cats," Hardy said of the number of feral, or wild, cats roaming Franklin County.
"They put the shelter population at risk," he said. "All of our cats are vaccinated as soon as they come in."
Kittens are more susceptible to the disease than adult cats, Hardy said.
Some people trap feral cats and bring them to veterinarians or animal shelters for neutering and vaccinations, then release them, he said. Hardy thinks it's a good practice that has been tried in other areas and has reduced the number of feral cats in an area. They're healthier and they don't reproduce, he said.
"Residents have to take more responsibility for homeless cats and dogs," Hardy said.
"Cat season," the time when most kittens are born, runs from early spring through the fall.
"This year we've been overwhelmed with cats," Hardy said.
The shelter still is taking in and adopting dogs.
It took in 363 dogs in the first six months of the year, 205 of which were adopted, he said.
"It doesn't mean that all those that were not adopted were euthanized," Hardy said.
Hardy checked the shelter's records and found that 62 of those not adopted were reclaimed by owners. A few special-breed dogs - Labrador and golden retrievers and German shepherds - were picked up by rescue groups, he said.
The shelter charges $100 to adopt a dog, including neutering and vaccinations. Cats cost $65.