Harrison is visited by a microbiologist who helps him take a close look at an invisible world and figure out how to stop viruses from hurting us. In the name of science, Harrison performs a top secret experiment at an elementary school to find out how quickly germs can spread. Viewers at home learn how to grow their own fungus.

The Big Question: "How does your nose get runny?" 1) MY GREAT CHALLENGE: Harrison tests school kids by seeing who among them can get their hands the cleanest. 2) IN THE FIELD: Roving reporter Sidney puts on a full containment suit and goes to a high-security lab where the most dangerous viruses in the world are studied. There, she discovers that not all germs are bad, and û surprise û we actually need some of them to survive!