Causes of Cat Scratch Disease (CSD)
Cat-scratch disease is caused by bacteria that cats carry in their saliva. They likely get the bacteria from fleas. They can spread it to their paws or fur when they lick themselves. You can get cat-scratch disease from a cat biting, scratching, or licking you. You can also get the bacteria in your eyes if you rub them after petting a cat that has the bacteria on its fur. Many people who get cat-scratch disease do not remember being scratched or bitten by a cat.

How cats and people become infected
Cats can get infected with B. henselae from flea bites and flea dirt (droppings) getting into their wounds. By scratching and biting at the fleas, cats pick up the infected flea dirt under their nails and between their teeth. Cats can also become infected by fighting with other cats that are infected. The germ spreads to people when infected cats bite or scratch a person hard enough to break their skin. The germ can also spread when infected cats lick at wounds or scabs that you may have.