What is the only thing that should differ between experimental treatments

If I understand you correctly; the two experimental treatments are the experimental condition and the control condition.

The thing differing between them should be what you want to examine, and especially the thing you want to know of how it influences something. So for example if you want to know how alcohol influences something, the difference should be that one condition will include alcohol and the other won't.

When a virus infects a host cell, it uses the cell's machinery to make copies of its own genetic material in a process called replication. finally, during the process of self-assembly, that protein shell and the copies of the virus's genetic material are assembled into new viruses.