What's really biohazardous?

Working in a lab I use a number of hazardous reagents. Most are chemical, but we've got bionasties too; things like genetically engineered bacteria, human cancer cells, cow blood, and replication deficient viruses. Although these things sound bad and all get labelled biohazard (the biohazard bags later get incinerated), the chances of picking up an infection from any of them is miniscule.

On the other hand, if one of us is sick and blows his nose, then there's a blob of snot full of a known human pathogen. To other humans, it's suddenly the most infectious and virulent thing in the lab. Of course, tissues go into the standard garbage.