Viruses vary considerably in size and shape. The smallest viruses are about 2 nanometers, while the largest viruses measure about 300 nanometers. Smallpox viruses are among the largest viruses; polio viruses are among the smallest.

Certain viruses contain ribonucleic acid (RNA), while others contain deoxyribonucleic acid (DNA). The nucleic acid portion of the viruses is known as the genome. The nucleic acid may be single-stranded or double-stranded; it may be linear or a closed loop; it may be continuous or occur in segments.