The 100 year thing is not as safe as it sounds. We learned a bit about that here in Richmond.
After Hurricanes Camille in 1969, the "can't happen again flood" of Agnes came just 3 years later, and was even more record setting.
"There is approximately a 63.4% chance of
one or more 100-year floods occurring in any 100-year period" (my italics.)
See
100 year flood math. Some would say something more than the 100 year flood line is the safe line
As for hantavirus, it is also known as Four Corners Disease
http://www.ncbi.nlm.nih.gov/pubmed/7711347 Four Corners Disease some history
Four Corners Disease history 
