What is the protein shell of a virus called?

The protein shell of a virus is called a capsid. It surrounds and protects the viral genome, helps the virus attach to or enter host cells, and can influence which organisms or tissues the virus infects.

Capsids are built from repeating protein units called protomers. These units assemble into larger visible structural arrangements known as capsomeres. The resulting shell commonly has helical or icosahedral symmetry, although bacteriophages and some other viruses have more elaborate shapes.

A common mix-up is confusing a capsid with an envelope. An envelope is a lipid membrane found around some viruses, usually acquired from a host cell during viral release; it may contain viral attachment proteins or “spikes.” The capsid remains the protein shell. In terminology, a nucleocapsid means the capsid together with the enclosed nucleic acid, and sometimes additional core proteins. Viruses without envelopes rely heavily on the capsid for both protection and host-cell attachment.