Viruses are microscopic entities that exist at the boundary between living and non-living matter. Despite their minuscule size, viruses have a profound impact on all forms of life, from bacteria and plants to humans and animals. They are the most abundant biological entities on Earth, outnumbering bacteria by a factor of ten. Viruses have played a key role in shaping ecosystems, driving evolution, and causing significant diseases in both humans and other organisms.

A virus is fundamentally a small particle, or "virion," composed of genetic material (either DNA or RNA) encased in a protein coat known as a capsid. Some viruses also have an outer lipid membrane called an envelope. Unlike bacteria or other microorganisms, viruses lack the machinery necessary for self-replication and metabolism. Instead, they must infect a host cell to reproduce. The virus hijacks the host's cellular machinery to replicate its genome and assemble new viral particles, which are then released to infect other cells.

The structure of viruses is incredibly diverse. There are simple, rod-shaped viruses, more complex spherical ones, and even geometrically intricate varieties like the icosahedral viruses. The genetic material within the virus—whether it is single-stranded or double-stranded, RNA or DNA—dictates the virus's replication strategy and determines how it interacts with the host organism. While some viruses infect only specific species or cell types, others are capable of infecting multiple hosts, sometimes crossing species barriers in the process.