You won't be, when you're through reading The Blue Death: Disease, Disaster, and the Water We Drink, by Robert Morris.
Sure, you might feel pretty secure when you're done with the first few chapters, about cholera outbreaks in London in the nineteenth century, and how a few intrepid souls (and one very dedicated doctor, John Snow) insisted that the disease was spread through the water, rather than through foggy and vague "miasmas" in the air.
You might even be feeling pretty secure after you read about how drinking water is chlorinated to kill all those tiny little bacteria and other disease-causing agents that we can't see. But when you get to the chapters in which Morris explains the 1993 cryptosporidium outbreak in Milwaukee, I'm guessing you might not be feeling so secure anymore--or thirsty.*
*Yeah, it's a scary book. But it's a really interesting one too. Give it a try!