[nudephil (OP)]

Here's an interesting question sent in by Riccardo:

Is it possible to hypothesise whether there have been any viruses that humans have ever 'suffered' from that have been beneficial to us?

I heard that about 8% of our genome is virus fragments. Could any of these resulted in, for example, a larger brain, ability to create verbal languages, better ability to disable attacks from other viruses and bacteria?

[Kryptid]

Some endogenous retroviruses have indeed been beneficial for us. Their genes have been found to be used in the placenta, for example: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6177113/

[Bored chemist]

The cowpox virus pretty much wiped out the smallpox virus.
It also demonstrated how vaccination might be used.

[evan_au]

Viruses (eg bacteriophages) make up part of your gut microbiome, and help prevent any one strain of bacterium from taking over. Before antibiotics were developed, bacteriophages were used to treat bacterial infections, mainly in Russia.
https://en.wikipedia.org/wiki/Bacteriophage

Some viruses are thought to contribute to "horizontal gene transfer" between species.
https://en.wikipedia.org/wiki/Horizontal_gene_transfer

There have been many cases where viruses carried by European colonists decimated the native population.
So it helped the colonists but destroyed the indigenous society; whether it was "beneficial" differs depending on which side you were on.

[evan_au]

Apparently, bacteriophages (viruses that prey on bacteria) are important in cheese making, controlling the mix of bacteria in the cheese.

That is towards the end of: https://www.sciencefriday.com/spotlights/cheese/