Why does a virus need a living cell to make more viruses?

A virus particle does not grow into a larger virus and split like a bacterium. New particles are assembled from components made inside a host cell. The virus carries genetic information, but it depends on the cell's working machinery and supplies to produce more of itself.

The process begins when a particle encounters a suitable cell. Molecules on the virus bind to particular molecules at the cell surface. That match helps determine which cells a virus can infect, although attachment alone is not enough: the cell must also support the later steps. Different viruses use different ways to cross or deliver their contents through the cell's boundary.

Once the genetic material becomes accessible, the infection redirects cellular processes. The cell's ribosomes, which normally assemble proteins from amino acids, can make viral proteins from the instructions provided during infection. Viral genetic material must also be copied. Some viruses bring or encode special copying enzymes because the cell does not already possess the exact machinery needed for that kind of genome.

Protein parts and genome copies then come together to form new particles. This is assembly, not a little parent virus giving birth. Many components are made separately and organized into structures that protect and transport the genetic material. Some particles gain a membrane envelope as they leave a cell, while others escape when the cell breaks open. The illustration shows a simplified break-open route, not the life cycle of every virus.

An infected cell may be damaged by diverted resources, disrupted functions or the release process. The body's response can also contribute to symptoms. Not every encounter produces a successful infection, and not every virus infects humans. Plants, animals, fungi and bacteria all have viruses associated with them.

This dependence explains why a virus cannot simply multiply on a bare tabletop: a surface does not provide functioning ribosomes, energy-processing systems and organized cell chemistry. A particle may remain capable of infection for some time under suitable conditions, but that is different from making new particles. The central mechanism is a transfer of instructions into an already working biological system, followed by the construction of new infectious packages.