Cruise ships often provide a framework for outbreaks of infectious diseases owing to their closed environment, contact between travelers from many countries, and crew transfers between ships.

During the initial stages of the pandemic, the Diamond Princess represented the setting of the largest outbreak outside mainland China.

On the cruise ship, transmission mainly occurred among passengers before the implementation of quarantine, while crew infections reached a peak after the quarantine.

On the Grand Princess cruise ship, crew members possibly got infected on voyage A and then transmitted the infection to passengers on voyage B.

The results of test done among passengers and crew on board the Diamond Princess showed 46.5% of asymptomatic infections at the time of testing which represents a quite high proportion.

Statistical models of the Diamond Princess outbreak suggest that 17.9% of infected persons never developed symptoms.

A high proportion of asymptomatic infections could explain, in part, the high attack rate among the passengers and crew.

On the Diamond Princess, SARS-CoV-2 RNA was identified on numerous surfaces in cabins of both symptomatic and asymptomatic infected passengers up to 17 days after cabins were vacated, but prior to disinfection procedures had been conducted (Takuya Yamagishi, National Institute of Infectious Diseases, personal communication, 2020). [CDC]

The reason behind the persistence of RNA for 17 days is not clearly known; was its central AC and humid atmosphere? The 23% infection rate in Diamond Princess could be explained by cohort quarantine.

This would explain my theory that central AC and humid airports are the hotspots and not the country itself.