Would you feel comfortable saying that a human who isn't bipedal is unnatural and abnormal? What if it there were such a person in your family?
Words matter.
Words matter.
We're talking about the language of science here. Biologists often talk about humans in terms of their biological characteristics. There is no judgement, only classification.
And acknowledging what is normal, is not a value judgement. It's not meant in a hurtful way.
Intent matters.
I don't think that's natural at all. "Natural" would be simply dying from cancer in your leg, after it metastasizes to the rest of your body. Amputation isn't natural, nor is any other human medicine. All medicine is unnatural, and working against what would happen in nature. Just putting a bandage on a small cut to avoid infection is unnatural: to be natural, you should let it get infected and then die from it.
Are you looking for some kind of deity to define English words for you?
That's irrelevant. There's nothing natural about extracting some chemical compounds from a plant on the other side of the world and using it to improve a medical problem you have.
>Many animals use their saliva to avoid infection.
Sure, that's fine. That's not the same as extracting chemical compounds from some plant somewhere else. Animals aren't doing anything like that.
Even ancient medicines, where they dug up some root somewhere and boiled it into a tea, is unnatural. Animals don't know how to do that, and even ancient humans had to figure it out with a long trial-and-error process that probably took thousands of years. They certainly didn't know about this stuff instinctively.
Saying "plant-based medicine is natural!" is like saying that concrete-and-steel buildings are "natural" because they're made with materials found in Earth's crust. It makes no sense at all.
Excepting "other side of the world", the rest is very natural. It is one of the methods of getting poisons for some organisms - eating plants that already contain them. For example, monarch caterpillars accumulate certain toxic glycosides acquired from plants, and use them to make themselves poisonous to predators. So do other insect species - feeding on plants toxic to their potential predators and then keeping the toxins is a common defense mechanism. One that humans borrowed too - e.g. poisonous dart frogs (people don't feed on them, usually, just take their poison, but the principle is similar).
Animals also been observed consuming certain plants when sick, though I am not sure about the efficiency of those methods. Chimps certainly are known to use some plants as anti-parasitic, etc. agents, and other simians too. There's evidence of elephants doing the same. I'm not sure where would we make a boundary between purposeful behavior of "smart" animals like chimps and pretty much the same behavior of humans, only at larger scale, but if we declare chimps being "natural", using plant medicines, at least a primitive ones, should be included.
Personally, I would not use ether unnatural or abnormal because they have negative connotations even though, by definition, they might be correct.
If I had to chose a word for this purpose today, I would chose the word atypical.
To my knowledge, there is not a negative connotation to the word atypical, but if we start using any word to refer to any specific group of people then the word we chose could, in time, develop its own connotations.