Epidemic
Turn a sickness into a weapon and it becomes an epidemic. This sickness spreads to everyone and it's all your fault because you put it in the water supply.
Imagine if countries decided to make sick weapons that would spread epidemics like this all over part of the world. Let's hope they don't...
えき
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These epidemics spread by 駅 (えき). Each train takes the sickness and shoots off in a different direction, spreading it further.
Picture a single 駅 on a map, bright red. Now watch the red spread to all of the other 駅 on the map, dyeing them red and spreading the epidemic all over your country.