I found some studies from India investigating the antiviral effects of belladonna, as a 200C homeopathic solution:
This one found belladonna reduced viral load in chick embryo brains after they were exposed to Japanese encephalitis virus. And
this one found that in chick eggs exposed to Japanese encephalitis virus, belladonna reduced the viral load in the brain, as well as increasing expression of TLR3, TLR7, TLR8, IFN-α and IFN-β.
However, both studies used belladonna 200C, and 200C is a
very diluted homeopathic solution, and definitely will not contain any molecules of the original substance (belladonna in this case). Any homeopathic solutions which are beyond the 12C dilution will not have any molecules of the original substance left in the solution.
So I question whether these studies are reliable.