A window into Enismirdal's mind

With what we know in this day and age about the impact of changing just one thing, the thinking man or woman would have all the best intentions of not changing a single thing. How do you cure a disease without causing population growth that can't be sustained without improving agricultural methods and animal husbandry? The problem arises that while that intention sounds well and good, real life is going to challenge it. What happens when the sick person is your child, your lover, your friend? Too easy to think "What would be the harm is just one little thing?"