Viruses are always much smaller and genetically simpler than bacteria.

The first viruses were discovered operationally: infectious agents small enough to pass through filters that trapped known bacteria. Their tiny size became part of the intuitive boundary between viruses and cellular life.

That assumption influenced what researchers looked for.

Mimivirus was so large that when it was found inside amoebae, it was initially mistaken for a bacterium. Genetic analysis eventually showed that it was a virus with an enormous particle and a genome far larger than those of many familiar viruses.

Other giant viruses followed, some with genomes exceeding a million base pairs and encoding hundreds of genes.

These discoveries did not erase the distinction between viruses and cells. Giant viruses still depend on host cells for reproduction and lack the full autonomous machinery of cellular organisms.

Most viruses also remain much smaller than bacteria.

What disappeared was the universal size rule. Particle diameter and genome size overlap enough that “smaller than bacteria” is no longer a reliable definition of what a virus is.