Viruses can be classified in several ways. The most useful is probably by the type of nucleic acid the virus contains, but they can also be classified:
by their geometry
by whether they have envelopes
by the identity of the host organism they can infect
by mode of transmission
by the type of disease they cause
Classification by type of constituent nucleic acid (i.e. by genome type), with examples:
... the average family size is 3.11.
In the village the population is spread out with 27.6% under the age of 18, 6.2% from 18 to 24, 17.7% from 25 to 44, 36.0% from 45 to 64, ...