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People are susceptible to infections. Once infected, some people recover after some characteristic time.
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The infection spreads within a city from person to person through physical proximity. Infected people recover after some characteristic time.
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The infection also spreads from city to city because people travel through air, road, or by trains. The arrows denote the traffic between various cities. Initially there is only one infected case in city D.
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After some time, there are infected cases in cities A and C. People are also getting recovered in these cities.
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Even though city D and city B are not connected, the infection reaches city B after some time.
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Eventually, the infection reaches all the cities. The first time since the outbreak when the active cases cross a threshold is used as a proxy for the risk associated with that city.
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Data for the three modes of transport - air, rail, and road - was collected/estimated by various methods.
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Bigger red circle indicates greater risk, while smaller red circle indicates lesser risk. In this example, city A carries more risk than city B.