You may have seen a cat testing the size of an opening by careful measurement with the head. Cats with white fur and skin on their ears are very prone to sunburn. Frequent sunburns can lead to skin cancer. Many white cats need surgery to remove all or part of a cancerous ear. Preventive measures include sunscreen, or better, keeping the cat indoors.
Cats can get tapeworms from eating fleas.
These worms live inside the cat forever, or until they are removed with medication. They reproduce by shedding a link from the end of their long bodies. This link crawls out the cat's anus, and sheds hundreds of eggs. These eggs are injested by flea larvae, and the cycles continues. Humans may get these tapeworms too, but only if they eat infected fleas.