I'm not quite sure I get your point. If Iran has proof that the US is responsible for that virus, then yes it can claim that the US engaged in an act of war. War does not work by one side committing an act of war and the other side getting in a free retaliation after, however. If Iran wants to retaliate, whether conventionally or not, the US also has the right to retaliate. So sure, Iran would have a legal rationale for attacking the US; the problem is that retaliating gives the US a legal rationale to invade Iran.

