Short answers, pulled from the story.
A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. It infects all life forms, including animals, plants, bacteria, and archaea, and it is the most numerous type of biological entity on Earth.
Dmitri Ivanovsky showed in 1892 that filtered sap from a diseased tobacco plant remained infectious despite removing all bacteria. In 1898 Martinus Beijerinck called that filtered infectious substance a virus, a moment considered the beginning of virology.
Scientists disagree. Viruses carry genetic material, reproduce, and evolve through natural selection, but they lack cell structure and their own metabolism, so they have been described as organisms at the edge of life and as replicators.
A virus forces a host cell to rapidly produce thousands of copies of itself, using the cell's machinery and metabolism because it is acellular. Its life cycle runs through six stages: attachment, penetration, uncoating, replication, assembly, and release.
The 1918 flu pandemic killed an estimated 40-50 million people, possibly as many as 100 million, and influenza caused four pandemics in the 20th century. HIV has been pandemic since at least the 1980s, and SARS-CoV-2 caused the COVID-19 pandemic beginning in 2020.
Vaccination is a cheap and effective way to prevent viral infections, and it eradicated smallpox and reduced illness from polio, measles, mumps, and rubella. Antiviral drugs, often nucleoside analogues such as aciclovir and lamivudine, interfere with viral replication.
Viruses are the most abundant biological entity in aquatic environments, with about ten million in a teaspoon of seawater. Most are bacteriophages and cyanophages that kill roughly 20 percent of marine microbial biomass each day and recycle carbon and nutrients through the viral shunt.