Malaseb is registered to treat Seborrhoeic Dermatitis and Dermatophytosis. Seborrhoeic Dermatitis is the scientific name for a skin infection involving bacteria (Staphylococcus) and yeast (Malassezia). Dermatophytosis is the scientific name for ringworm, a condition which is not due to worms (like the name suggests) but is in fact a fungal infection.
It is normal in dogs to have low numbers of resident bacteria and yeasts on the skins surface.