I don't know the precise reasoning that would be used in argument, but I think your first hurdle is picking a respondent/defendant. Without the ability to pinpoint exactly when, where, and who caused the transmission of the virus, who is the action against? Assuming you pick a person, how do you isolate them as the cause rather than anything else you've come into contact with - especially given Covid-19 is sheddable when asymptomatic. Add to that the aspect that if the person is symptomatic and to such an extent that you are demonstrably aware, there's the issues of consent and/or contributory negligence where it could be argued that you were willingly in that person's presence and can be inferred to be aware of the potential risks.
In terms of suing another country, that's a whole other issue - and beyond my pay grade. Countries are sovereign and not subject to the jurisdictions of the courts of another country unless they agree to be (such as via treaty). I doubt you'll catch any country agreeing to be bound be the decisions of a court of a different country finding the first country liable for a disease.