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A virus consists of a nucleic acid core—either DNA or RNA—enclosed in a protective protein shell called a capsid. Some viruses have an additional lipid envelope derived from the host cell membrane that incorporates viral proteins. This simple structure allows viruses to efficiently invade host cells and use their machinery for replication.

Enveloped viruses use specialized surface proteins or glycoproteins to recognize and attach to specific receptors on host cells, often fusing their lipid envelope with the host cell membrane. Non-enveloped viruses rely on capsid proteins for attachment and entry. Both strategies enable viruses to deliver their genetic material into the host cell.

HIV, an RNA virus, uses the enzyme reverse transcriptase to convert its RNA genome into DNA, known as a provirus. This DNA integrates into the host genome, enabling persistent infection and replication. This mechanism allows HIV to establish long-term infection by becoming part of the host's genetic material.

Bacteriophages possess complex structures including a capsid, tail sheath, and tail fibers that enable them to inject genetic material directly into bacterial cells. These specialized components allow bacteriophages to penetrate the bacterial cell wall and deliver their DNA into the cytoplasm, facilitating infection of bacteria.

Viruses are obligate intracellular parasites because they rely entirely on host cells for replication and assembly of new viral particles. They lack their own metabolic machinery and cellular structures, so they must hijack the host cell's resources and enzymes to reproduce. Without a host cell, viruses cannot replicate independently.

Viral replication often disrupts host cellular functions, leading to cell damage, apoptosis, or lysis. In multicellular organisms, this disruption can result in various diseases ranging from mild infections to severe conditions such as AIDS and cancer. The extent of damage depends on the virus type and the infected cell's ability to resist viral takeover.

DNA viruses such as herpesviruses encode their own replication enzymes, including DNA polymerase, to supplement the host's transcriptional machinery. This allows them to express viral genes and replicate their DNA independently. In contrast, RNA viruses like HIV must convert their RNA to DNA or use host machinery differently to achieve replication.