A virus is an infectious agent that can only replicate within a host organism. Viruses can infect a variety of living organisms, including bacteria, plants, and animals. Viruses are so small that a microscope is necessary to visualize them, and they have a very simple structure. When a virus particle is independent from its host, it consists of a viral genome, or genetic material, contained within a protein shell called a capsid. In some viruses, the protein shell is enclosed in a membrane called an envelope. Viral genomes are very diverse, since they can be DNA or RNA, single- or double-stranded, linear or circular, and vary in length and in the number of DNA or RNA molecules.
The viral replication process begins when a virus infects its host by attaching to the host cell and penetrating the cell wall or membrane. The virus’s genome is uncoated from the protein and injected into the host cell. Then the viral genome hijacks the host cell’s machinery, forcing it to replicate the viral genome and produce viral proteins to make new capsids. Next, the viral particles are assembled into new viruses. The new viruses burst out of the host cell during a process called lysis, which kills the host cell. Some viruses take a portion of the host’s membrane during the lysis process to form an envelope around the capsid.
Following viral replication, the new viruses may go on to infect new hosts. Many viruses cause diseases in humans, such as influenza, chicken pox, AIDS, the common cold, and rabies. The primary way to prevent viral infections is vaccination, which administers a vaccine made of inactive viral particles to an unaffected individual, in order to increase the individual’s immunity to the disease.