Why are there no infected transports between two 100% infected countries?

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I regularly have trouble infecting some islands (Madagascar, New Zealand, Caribbean).
In most cases almost the entire world is infected, and ships and planes are flying around (plague not detected). But how is it possible that between two 100% infected countries, there are healthy planes and ships?
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Sources: Stack Exchange - This article follows the attribution requirements of Stack Exchange and is licensed under CC BY-SA 3.0.
Images: Ron Lach, Edward Jenner, CDC, Edward Jenner