Bartonella henselae is the most common bacterial cause of cat scratch disease. Cats often carry it without symptoms, and kittens may have higher carriage rates. Cats typically acquire the bacterium through flea exposure, making flea control an important prevention step. In humans, the bacterium can enter through a scratch or bite and lead to lymph node swelling and fever. In immunocompromised individuals, broader involvement is more likely, so clinical evaluation is important when symptoms occur.