This is a good question. The answer is pretty definitive, that yes, this is a thing that superpowers have been confirmed to do in the past. There's much less to go on about current work, but we can look at the type of specialized equipment that is getting built or purchased by various nations and make inferences, and trust that intelligence agencies are making substantiated conclusions. To excerpt from a Congressional Research Service report[0] in 2008:
"No nation publicly acknowledges either an offensive biological weapons (BW) program or stockpile. Examination of unclassified sources indicates that several nations are considered, with varying degrees of certainty, to have some BW capability. These are: China, Cuba, Egypt, Iran, Israel, North Korea, Russia, Syria, and Taiwan."
The most data exists on the Soviet Union. Defectors from the soviet union had a lot to say about Biopreparat.[1] Not all of the rumors have been substantiated, but they did have a number of incidents that strongly suggest they had the souped up versions of several agents defectors claimed they did like anthrax[2], smallpox[3] and marburg[4]. In particular, the smallpox incident[3] that occurred had a high percentage of cases that progressed to the hemorrhagic form which indicates unusual virility in the strain.
There is also evidence of at least ongoing research on those agents. One researcher at vector died in 2004 from pricking herself with a needle filled with Ebola[5]. Whether that's offensive in nature is very difficult to determine. However there's definitely some transparency issues that have increased with time at this facility after a change in leadership in ~2004 or 2005.
[0] http://usiraq.procon.org/sourcefiles/CRS_2-20-08.pdf
[1] https://en.wikipedia.org/wiki/Biopreparat
[2] https://en.wikipedia.org/wiki/Sverdlovsk_anthrax_leak
[3] https://en.wikipedia.org/wiki/Aral_smallpox_incident
[4] Placeholder. Having a hard time finding where the info on Dr. Ustinov came from. This might be worth a read: https://weaponews.com/history/65348565-strain-u-the-tragedy-...
[5] https://www.nytimes.com/2004/05/25/world/russian-scientist-d...
Most subject matter experts seem pretty convinced this is happening.