A capsid is the protein shell that encases and protects the genetic material of a virus. It is composed of protein subunits called capsomers, which assemble in specific patterns to form a protective layer. The structure of a capsid can vary widely among different viruses, influencing how they infect host cells. Capsids play a crucial role in the life cycle of a virus, as they help deliver the viral genome into host cells. They can also aid in the recognition and attachment to host cell receptors, facilitating the entry of the virus. Understanding capsids is essential for developing antiviral treatments and vaccines against various viruses, including influenza and HIV.