A capsid is the protein shell that encases and protects the genetic material of a virus. It is made up of protein subunits called capsomers, which assemble in specific patterns to form a protective layer. The structure of the capsid can vary widely among different viruses, influencing how they infect host cells. The primary function of the capsid is to safeguard the viral genome, which can be either DNA or RNA. Additionally, the capsid plays a crucial role in the virus's ability to attach to and enter host cells, making it essential for the virus's life cycle and propagation.