Viruses are microscopic parasites, generally much smaller than bacteria. They lack the capacity to thrive and reproduce outside of a host body but can lie dormant on objects waiting to be picked up by a host or be breathed in from the air.

Predominantly, viruses have a reputation for being the cause of contagion.

A virus is a biological agent that reproduces inside the cells of living hosts. When infected by a virus, a host cell is forced to produce many thousands of identical copies of the original virus at an extraordinary rate.

For example, when suffering from the common cold, one sneeze emits 20,000 droplets containing rhinovirus or coronavirus particles. Touching or breathing those droplets in, is all it takes for a cold to spread.

It wasn’t until 1898 when the presence of viruses was acknowledged.  It would take a few more decades before anyone actually saw a virus. Once the electron microscope was developed in 1931 by German scientists Ernst Ruska and Max Knoll, the first virus could be visualized with the new high resolution technology.

With better methods to sequence and compare viral genomes, and with the constant influx of new scientific data, what we know about viruses and their histories is constantly being fine-tuned. 

Most viruses are extremely tiny in size compared to bacteria. They are usually measured in nanometers. For example, the polio virus, 30 nm across, is about 10,000 times smaller than a grain of salt.

Until 1992, the notion that viruses were much smaller than bacteria, with tiny genomes was taken for granted. That year scientists discovered a bacteria-like structure within some amoebae in a water-cooling tower.

As it turns out, what they discovered was not a bacterial species, but a very large virus, which they dubbed Mimivirus. The virus is about 750 nm in size and may also have the same staining properties as gram-positive bacteria.

This was followed by the discovery of other large viruses such as the Mamavirus and Megavirus. 

It is not known how these large viruses evolved. They may be degenerate cells, which have become parasites of other cells (Mimiviruses infect amoeba), or they may be more typical viruses that keep acquiring additional host genes.

Mimiviruses require a host’s cellular machinery to produce proteins, just like other smaller viruses. However, their genome still contains many remnants of genes associated with the process of translation.

Such discoveries bring up new questions and open new avenues of research. In the future these studies may provide answers to fundamental questions about the origins of viruses, how they reached their present parasitic state, and whether viruses should be included in the tree of life.

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Adenoviruses can cause a wide range of illnesses

Adenoviruses rarely cause serious illness or death. However, infants and people with weakened immune systems, or existing respiratory or cardiac disease, are at higher risk of developing severe illness from an adenovirus infection.