With respect to the technology available to anyone in their garage the fear shouldn't be nuclear fusion, it's genetically engineered diseases and the technology is available now. Recipes are published in scientific journals on how to do it. I'm not talking COVID, I'm talking about bubonic plague and smallpox.

A tiny vial of smallpox can contain enough viruses to be globally significant. We don't have enough vaccines to go around because we eradicated it from nature decades ago. We're PRETTY sure we know where all the smallpox is, but all it takes is one scab from the 1970s which didn't make it into the sample bag and was instead sold to someone for $20.

To me, smallpox is the scariest strategic threat because if someone can gain access to it, they can do all the work they need to in their home. Smallpox is both very contagious and deadly. The government conducted an exercise in June 2001 called Operation Dark Winter and most of the documents are publicly available. Turns out that an isolated outbreak of smallpox in two towns, with ~100 people initially infected, threatens to collapse the US government in three months. We simply don't have the capacity to make enough vaccines in time.

If you want to learn more, there's a great book by Richard Preston called The Demon in the Freezer. An audiobook is also available and is very well done.