I hear it all of the time. "I don't want to take that medicine because it's not natural". "This food is not natural". "I choose to live this way because it's natural". "This is good because it's natural". "This is bad because it's artificial".
Actually, we live in a world that is not "natural". It's difficult to define "natural" but many people seem to think that it's how were "intended" to be. The "artificial" character of medicine, vaccine, and food processing somehow indicates that these are harmful. The fact that we evolved in a world that wasn't human-made, somehow indicates that we are not "designed" for the modern world.
Think of dogs. Chihuahuas would not survive in the wild that their ancestors occupied. Neither would poodles. Neither would most domestic animals. Neither would we. "Natural" is not an option.
Actually, benefit and harm are both independent of "natural" vs. "artificial", and there is almost nothing left that is not modified already by human activity. The climate, CO2 content of the air, earth's temperature, the substances that waft about in the air and water, and permeate our bodies, have all been created or modified by human activity. Our brains and our relationships have been modified by human activity. All ecosystems have been significantly impacted by human activity, with intended and unintended consequences, beneficial and harmful depending on your perspective. So have our bodies. Our predecessors and ancestors had much smaller build. They were shorter and slimmer. Obesity either did not exist, or was rare, in ancient times. Lives were also shorter - much shorter. Life was also brutal. Ancient diseases are now almost forgotten in Western society - polio, leprosy, cholera, bubonic plague, tetanus, smallpox, typhoid fever, gangrene. Many aspects of life are immeasurably better, and it's not "natural."
Exercise is good for us, but not because ancient people had to work hard. Lack of dietary excess is good for us, but not because ancient people were thin. Sanitation is good for us, and ancient people who had sanitary systems likely benefited as well, but humans evolved in a presanitation world.
Humans by Era Average Lifespan at Birth (wikipedia
Upper Paleolithic 33 At age 15: 39 (to age 54)
Bronze Age 18
Bronze age, Sweden40-60
Classical Greece 20-30
Classical Rome 20-30
Pre-Columbian North America 25-30
Medieval Islamic Caliphate 35+ The average lifespans of the elite class were 59–84.3 years in the Middle East and 69–75 in Islamic Spain.
Medieval Britain 20-30
Early 20th Century 30-45
Current world average 66.57 2009 est.
There is no evidence that any "natural" activity of ancient times was beneficial to quality or quantity of life, because they simply did not live that long. Not their food, not their farming methods, not their medical technology, including Indian and Chinese herbal remedies.
So we are unnatural creatures in an unnatural world. That doesn't mean that pesticides are good (or bad) or fertilizers are good (or bad) or medicines are good (or bad) or SUVs are good (or bad) or processed food is good (or bad) or vegetarianism/omnivory is good (or bad). The utility, benefit / cost of these substances and activity is independent of their "naturalness", and depends on how they are used, in what circumstances, and to what degree, and their interrelation to the rest of their world, local and global. I think that many of the excesses of modern industrialized, commercial life are very harmful, but it's not because they are unnatural.
Sometimes, when people tell me "I don't want to take this cholesterol pill. It's not natural" I tell them "Neither is living past 30. If nature was all that we had to go by, you would be dead already, and so would your kids."