The TB bacteria is indeed airborn, and can't be transmitted through food - but can be transmitted through the food-HANDLERS and anyone who comes into contact with them via their cough, sneeze, or any mucus droplet.
Although no guarantee, if this 'exchange student' (or any other immigrant worker) had been properly screened, TB should have been detected if infected upon entry to this country. All LEGAL immigrants to the U.S. are required by law to have a chest X-ray and be up-to-date on immunizations to prevent this from happening. ONE of many reasons immigration laws are in place to protect U.S. citizens and legal residents.
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