Parasites can be internal parasites which you cannot see like roundworms/tapeworms/hookworms etc or they can be ectoparasites like fleas/ticks/lice which can affect your pet’s health. There are some ectoparasites which you still cannot see like “mites” and it becomes very important to keep your pets up to date on parasite prevention as many parasites are zoonotic ( transmissible to humans )