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Viruses cannot reproduce independently; they hijack host cell machinery to replicate. After entering a cell, the virus uncoats and directs the host's transcription and translation systems to copy its genetic material and synthesize viral proteins. The host cell then assembles these components into thousands of new viral progeny, which can rupture and kill the host cell or be released through budding.

All viruses contain a viral genome made of single or double-stranded RNA or DNA, enclosed in a protective protein coat called the capsid. Together, these form the nucleocapsid. Many eukaryotic viruses have an additional phospholipid envelope surrounding the capsid, often derived from infected host cell membranes and embedded with viral proteins. Some animal viruses also contain a viral tegument, a protein cluster between the envelope and capsid.

Surface proteins on viruses recognize and bind to specific receptors on host cells. Some viruses use these proteins to trigger endocytosis, while envelope-coated viruses can directly fuse with the host cell membrane. Bacteriophages may inject their genome directly into the host without entering the cell. Once inside or injected, the virus is uncoated, releasing its genetic material to begin replication.

In the lytic cycle, new viruses burst out of the host cell, killing it in the process. In the lysogenic cycle, viral DNA integrates into the host genome where it remains dormant and is copied each time the host cell replicates. The lysogenic cycle of bacteriophages allows viruses to persist without immediately destroying the host, whereas the lytic cycle produces rapid viral progeny release.

Viruses infect bacteria, plants, and animals, causing diverse diseases in humans ranging from the common cold to hepatitis and smallpox. Plant viruses cause epidemics leading to massive crop and economic losses. Bacteriophages are studied for phage therapy to treat multidrug-resistant bacterial infections. Understanding viral biology is essential for developing treatments and preventing outbreaks across veterinary, agricultural, and medical fields.

Viruses possess specialized surface proteins that enable them to recognize and bind to specific receptors on target cells. These proteins are adapted to attack particular cell types, making viral infection highly selective. The specificity of these surface proteins determines which host cells a virus can infect, whether bacteria, plant cells, or animal cells, and is central to the viral attachment stage of infection.

The viral genome, composed of RNA or DNA, carries the genetic instructions for producing viral proteins and replicating the virus. Once inside the host cell, the genome is transcribed and translated using the host's cellular machinery. The genome encodes the proteins that form the capsid and other viral components, directing the host cell to manufacture thousands of new viral particles from a single infection.