Viruses are infectious agents that cannot grow or reproduce apart from a living cell. They are substantially smaller than bacteria and cannot be removed by the typical sterilizing grade filter (see Sterilizing Filtration of Liquids 〈1229.4〉). Viruses are composed of genetic material, either DNA or RNA, and are surrounded by a protein coat and, in some viruses, by a membranous envelope. Viruses can introduce their DNA/RNA into living cells where they can replicate to spread the infection. Viruses cause a variety of diseases, e.g., common cold, influenza, warts, AIDS, Ebola, and smallpox. To minimize the risk to patients, sterile materials must be produced in a manner that minimizes the potential for viral contamination. Detailed treatment of this subject can be found in Viral Safety Evaluation of Biotechnology Products Derived from Cell Lines of Human or Animal Origin 〈1050〉 and Design, Evaluation, and Characterization of Viral Clearance Procedures 〈1050.1〉.