Don’t worry! Peanuts contain plenty of healthy monounsaturated fat and can be a good snack, and are also a good source of niacin. But they are often mistakenly grouped with nuts when they are actually legumes.

The downside to peanuts is that some people are highly allergic to them, and that they may contain aflatoxins- but this appears to be much less of a concern than it used to be:

http://en.wikipedia.org/wiki/Peanut
“Peanuts and aflatoxin

Peanuts may be contaminated with the mold Aspergillus flavus which produces a carcinogenic substance called aflatoxin. Lower quality specimens, particularly where mold is evident, are more likely to be contaminated.[30] USDA tests every truckload of raw peanuts for aflatoxin, any containing aflatoxin levels of more than 20 parts per billion are destroyed. The Peanut Industry has manufacturing steps in place to ensure all peanuts are inspected for aflatoxin. Peanuts are also processed at a high temperature to ensure any microorganisms are killed.”

Cheers,
Kevin