|MadSci Network: Virology|
Good question, and a very simple answer to it : YES!
The size of viruses range from 10nm for the smallest to 500nm for the biggest (nm = nanometer = 1/1,000,000,000 of a meter). The average human cell size is 100um (um = micrometer = 1/1,000,000 of a meter)which means that a virus is 200 to 10 000 times smaller than an average human cell. As a better way to understand the scale of this, imagine a pin head (2 millimeters) beside a 6 stories building (20 meters).
Capillaries cannot keep such small things from "leaking" since they are designed for letting some proteins, gases and other small molecules leak.
I hope this answered your question,
Try the links in the MadSci Library for more information on Virology.