Some plants are lethal even though they are natural. But if you die from eating a lethal but natural plant, you will not be said to have died of natural causes. Ditto, if you die from being bitten by a poisonous snake whose venom is quite natural. If you die from lung cancer caused by smoking tobacco, a natural plant, you will, however, be said to have died of natural causes. Fleas on dogs are natural.
Flea collars are unnatural.
Mosquitoes and flies are natural, though most people find them to be a nuisance and prefer the unnatural comfort of mosquitoeless nights and flyless barbecues. Eating meat might be a natural act, but eating cooked meat is unnatural. Most sauces put on meat are made with both natural and artificial ingredients. Salt is natural, but some healthy people avoid salt like the plague. Civilization is unnatural.