If there's a way to cause injury that won't fully manifest until after they've returned to the US, that'd be better.

Thats why you hide raw meat in a dish that won't kill worms / parasites and give it to them, especially meats that are known to have parasites that stay in the body undetected and multiply before causing isses

Bonus points if you marinate the raw meat in forest floor dirt for the day

That'd be really expensive in american healthcare