1.1 What is a Virus?

A virus is a microscopic infectious agent that cannot reproduce independently. Unlike living cells, viruses do not have the full machinery needed to produce energy, grow, or divide. Instead, they must enter a host cell and “use” the host’s internal systems to create new virus particles.

Viruses can infect many types of organisms, including animals, plants, fungi, and bacteria. Although simple in structure, viruses can spread efficiently and evolve quickly, which is why they are important in biology, biotechnology, and public health.

^{Figure 1. Viruses example. Source:
https://openstax.org/books/biology-2e/pages/21-1-viral-evolution-morphology-and-classification}