For one thing, most countries are not self sufficient in basic necessities (not even food!). You could do it for a very short time, but not sustain it. Also, the economy of the country depends on international trade. Modern economies actually don't have a lot of slack. If you shut down trade, quite a few companies will just go under. You would have to back that up with huge subsidies. It would be a massive undertaking.
Viruses don't live long enough to survive ocean shipping, so as long as the (tiny) crew is kept isolated, ocean shipping is fine. Within a country, it's not hard to disinfect a truck and keep the driver isolated.
Of course, if the originating factory is shut, then you're screwed - but that's true regardless of trade.
Also worth noting: it takes a tiny number of people to produce tons of food (calories). Maybe not true for artisanal salami but for bulk basics.
The US is the single largest exporter of grain in the world. Not the largest producer, not the largest consumer, but the largest exporter.
And the US farming population is ~1% of the total US pop. Modern tractors and combines are amazing.
Doesn't mean there aren't a ton of bodies involved in processing and transportation, though. You can probably find local-ish wheat in the US, but you're not going to boil and eat the wheat berries for a porridge like it's 1850 (though you could); you're going to buy breakfast cereal.
Well, if there's a widespread virus that has "the power to kill a perfectly healthy young man or woman within 24 hours of showing the first signs of infection" the "economy" and "international trade" would be the least of our worries...
