Rats were most common in the modern, newly built areas of Hanoi because the modern and apparently hygienic sewage system provided ideal breeding grounds for rodents apart from being a good transport system as well, for the rats. Sewage from the old city was drained out into the river or overflowed in the streets during heavy rains. The large sewers now became a protected breeding and living space for rats. Hanoi, despite its “modernity”, became the chief cause of the plague in 1903.
