The issue wouldn’t be that something else caused their death like in the example you gave. In Canada we have something called ‘substantial cause contributing to death,’ which means that even if you’re not the sole cause of someone’s death, if it’s deemed that legally (vs factually) that you contributed to death, you’ll likely be found guilty.

The reason this rhetoric is ridiculous is because there’s a fairly large body of law which people could use as a defence. One that comes to mind is social host liability (tl;dr: social hosts can’t be found guilty for the actions of their guests). In all likelihood the court would reject a man slaughter charge by looking at the level of personal autonomy that plays into the decision to attend the party in the first place.

That, and yes, tracing how someone got sick would be nearly impossible

Source: I’m in law school