Viruses are tiny infectious agents that consist of genetic material, either DNA or RNA, surrounded by a protective protein coat called a capsid. Some viruses also have an outer lipid envelope, which helps them enter host cells. The structure of a virus is relatively simple compared to living organisms, as they lack cellular machinery and cannot reproduce on their own. The shape of viruses can vary widely, with common forms including helical, icosahedral, and complex structures. The specific arrangement of proteins in the capsid determines the virus's ability to infect specific host cells. Understanding virus structure is crucial for developing vaccines and antiviral treatments.