The black residue in the cat's body hair may be the residue of fleas. It can be dewormed and cleaned externally. If it does not work, it may be ringworm. The cat should be isolated and sent to the pet hospital for examination. After confirming the cause of the disease, targeted treatment is required. During this period, the cat's living environment and used items must be thoroughly cleaned and disinfected to prevent the occurrence of bacteria that may affect the cat's health.