Viral structure refers to the physical composition of a virus, which is made up of genetic material and a protective protein coat called a capsid. The genetic material can be either DNA or RNA, and it carries the information necessary for the virus to replicate and infect host cells. Some viruses also have an outer lipid envelope that helps them enter host cells. The shape of viruses can vary widely, with common forms including icosahedral, helical, and complex structures. The specific arrangement of proteins in the capsid and the presence of surface proteins, known as spike proteins, play crucial roles in how viruses attach to and invade host cells.