Can you buy travel medical insurance after being diagnosed with any medical condition after already arriving in the USA?

Unfortunately if you're diagnosed with a condition while in the US and you don't already have travel insurance it will be considered a pre-existing condition.Pre-existing conditions are not covered under any type of travel medical insurance due to the short-term nature of the plans. The only thing that is covered in relation to a pre-existing condition is something diagnosed as an acute onset of a pre-existing condition. An acute onset of a pre-existing condition is a sudden and unexpected recurrence of a pre-existing condition and is up to the doctors diagnosis. Customarily, travel medical insurance only covers anything new that occurs, emergency and non-emergency alike; like a new illness, sickness, or injury that happens after the policy starts.