The Parvoviridae family includes the smallest known viruses, and some of the most environmentally resistant.

In some cases, very small viruses will not be filtered out.

It is not clear that all small viruses have originated from more complex viruses by means of genome size reduction.

In 1956, Crick and Watson speculated on the structure of small viruses.

An attogram, very roughly, is one tenth the weight of a small virus.

An in vitro evaluation of condoms as barriers to a small virus.

The filters capture particles smaller than the smallest virus, with officials saying they catch 99.97 percent of particulates.

"It may be a small virus that may be over in 24 hours," Lukas said.

Enteroviruses are small viruses that live in the intestines of infected humans or animals.

Scientists presumed it was a small virus that they could not culture.