Viruses are tiny infectious agents that can only replicate inside the living cells of an organism. Unlike bacteria, viruses are not considered living organisms as they lack the basic structures needed for independent survival, such as the ability to produce energy or carry out metabolic processes. Viruses consist of genetic material, either DNA or RNA, enclosed in a protein coat called a capsid. They can infect all forms of life, including animals, plants, and microorganisms like bacteria. Once a virus enters a host cell, it hijacks the cell’s machinery to produce more virus particles, often leading to the destruction of the host cell. Some viruses cause diseases like the common cold, flu, or more serious conditions like HIV and COVID-19, while others may remain dormant in the body for years. Understanding viruses is crucial for developing vaccines and antiviral treatments to combat the various diseases they cause.