Viral size matters

1 June 2010

One property of viruses that is difficult to conceptualize is their small size. I can tell you that viruses can be anywhere from 20 to 750 nanometers in diameter, but that’s not easy to visualize, even for those of us who routinely work with small measurements. One way to demonstrate how small viruses are is to compare them with animal and plant cells, bacteria, proteins, molecules, and atoms, as shown below:

But even comparisons of this type fall short because they do not provide a readily grasped real-life reference. A better way was suggested by my colleague Karla Kirkegaard: Start by multiplying the size of viruses and humans one million times. A supine human would then extend from California to Colorado. At this scale, a cell would be about the size of a lecture hall. Depending on their size, viruses would either be lemons (poliovirus, 30 nanometers), grapefruits (retroviruses, 100 nanometers), or watermelons (poxvirus, 250 nanometers).

Peter Palese has a different way of relating the small size of viruses. If you magnify a virus so it is the size of a human fist, then a cell would be about half the size of the Empire State Building.