It's not just parvo either!
You can also get any of these:
Roundworm (and any other intestinal worms), Leptospira (sp?) or Leptospirosis, Giardia, Coccadia, Hydatid disease, Campylobacter, Cryptosporidium, E. Coli and I think there are a few others too.
I never let Batty sniff at other dogs poop when we're on a walk because of these - it may seem silly, but I'd rather be safe! People can catch these too!