The American point is true. Look up where reported cases of Malaria are in the USA and you will get random locations throughout many central and more northern states. Malaria can't survive in colder temperatures so while it might be actually active in the souther edges of the USA any reports much further north are the result of travel where someone caught it while abroad but didn't notice until having already returned.

At least with something like Malaria it can't then be transmitted to others in those colder areas as again the cause of Malaria can't survive there, and the required mosquito won't be present, but anything with other means of person to person transmission will persist and spread, like STDs.