Sneezing is actually the most common sign of viral respiratory infections in cats.
They’re usually caused by either the herpes virus or the calicivirus, two very common bugs that like to hang around in places like overcrowded cat shelters.
Usually, once the cat is removed from overcrowded conditions their own immune systems defeat these bugs, but sometimes cats need supportive care to help them feel better and get well faster. Keeping the nose clear of debris and providing a humidified environment can make them more comfortable. Providing a more “smelly” diet, such as canned cat food that’s been warmed slightly to enhance the aroma, will encourage them to eat more, even though they can’t smell their food as well.