CLAYTON:
And speaking of vaccinations and immunity and so on, think about this: Native populations in South America, North America and Australia, they were wiped out by disease. -I shouldn't say wiped out, but greatly lessened by disease. What was the number one disease that Europeans gave these native people?

ARIA: Smallpox, I think.

MERIAH: Yeah, smallpox.

CLAYTON:
Everybody says smallpox. It was measles. Stop and think about it: you have smallpox, and you're an explorer - a conquistador. If you had smallpox, you would be too sick to get off the ship. But if you had the measles… You know, you've been packing 30 pounds of armor around all your life. What is fever or a little rash to you?

ARIA: A little discomfort.

CLAYTON:
It was measles that killed them, a very simple innocuous disease. Like you said, a little discomfort. But their immune systems had never encountered that virus before, and it killed them by the hundreds.

MERIAH:
Because they got it when they older, and not when they were kids, do you think?

CLAYTON:
There is some belief that may have been the case, but the children died just as quickly as the adults did. Are we doing the same thing to ourselves? You know, are we so compromising our immune systems now, that some little bacteria that's lived in some stream sediments for thousands and thousands of years, and which our grandparents, our great-grandparents, our great-great-grandparents drank with immunity, will suddenly now become a violent killer, but our grandparents had the natural immune system to deal with it?

So that's why I'm saying, regarding your Footprint Island: your most efficient way of dealing with these water quality issues is to keep your watershed intact, be careful who's living in it, but go ahead and drink the water.