One of the main reasons for the slow population growth was the low productivity, inefficient harvesting, and the prospect of famine. Under William's leadership, the introduction of new crops, the development of wind and water resources, and the application of technologies such as the yoke and mouldboard plough all greatly stimulated the development of agriculture.

The use of newly issued currency in the Norman kingdom replaced the primitive barter trade, facilitating the development of commerce and the formation of towns, and with it, the population grew and gathered.

But agricultural production has not kept pace with population growth, and the disparity between the rich and the poor has led to frequent famines in various places over the years, with the result that poverty and hunger have surrounded every crowded and dirty town.

It is common for people in the city to live with big black rats every day; Not only that, but people and rats are also in close contact on cargo ships that travel to and from the city; As long as there are people, there are rats. These black rats are the most vulnerable targets of the plague, and when they die, the humans on board are naturally the next targets.

This is not just the case in Normandy, but in the affluent Flanders and Anjou. These areas were already very prosperous, with developed trade, dense population, and a very large number of towns, coupled with the development of the commercial economy of the Norman kingdom in recent years, a large number of people poured into the cities, resulting in congestion and chaos in the cities.

However, most of these cities were self-governing, and with the exception of a few large cities that were run by officials sent by William, most of them were still governed by city councils composed of nobles, merchants, and gentry landowners, who did not have the same strong executive power as the cities of the Duchy of Normandy, and were defenseless against the death scythe of the Black Death.

So far, however, its claws have not been able to capture a single high-ranking cleric or nobleman, as these men have adopted the most effective tactic against the plague at the time: the Thirty-Six Schemes, and the Thirty-Six Schemes. While large numbers of poor people who could not afford transportation were forced to stay in the epidemic areas and wait to die, they traveled in ornate horse-drawn carriages to and from the famous and clean hot springs. The only Duchy of Normandy that did not have a large-scale plague outbreak became their ideal location.

After William's return, the Duchy of Normandy launched a vigorous rodent extermination operation under his auspices.

After the third large-scale outbreak of plague in southern China, Pasteur sent an assistant, Alexandre Brown, a member of the Swiss-born French colonial medical team. Alexandre Yersin (1863-1943) traveled to Hong Kong to collect and isolate the germs of the plague. When Yersin arrived in Hong Kong, he was only assigned to work in the corridor outside the ward, where all the experimental equipment and experimental animals he brought were kept.

At first, he was not allowed to visit the morgue, but after checking up and down, he was allowed to stay in the morgue for a few minutes. In the intervening minutes, Yersin examined a freshly dead sailor, who used a sterile needle to pick up a lump on the inner thigh of the corpse and extract some fluid. He then observed the liquid sample under a microscope and inoculated it into the guinea pigs he had brought in, sending the remaining sample back to the Institut Pasteur in Paris.

On June 24, 1894, Yersin wrote to Pasteur to report that the bacteria in the liquid were rod-shaped bacteria, which had tested negative on the Gram test, and he finally asserted, "There is no doubt that this is the microorganism that causes the plague!" A few days later, the guinea pigs inoculated with the germ died, and Yersin isolated the same bacteria from them. Yersin had always wondered where the large number of dead rats on the streets of Hong Kong, in hospitals and in the corridors came from, and when he did the same with them, he found that the bacteria on them were the same as those on sailors, and he judged that the bacteria could infect both humans and rodents.

Around this time, Robert? Koch was also convinced that the plague in southern China was caused by microorganisms, so he sent his Japanese colleague Shibasaburo Kitasato (1853-1931) and a large group of assistants and equipment to investigate the matter. Kitasato Shibazaburo took a sample of bacteria from a sailor's finger, but when observed, it was found to be gram-positive, and Shibazaburo failed to prove that his bacteria were capable of causing the same disease in humans and animals. Fate may have played a joke on Shiba Saburo, resulting in the germ he extracted was not exactly the plague bacterium he was looking for. At this point, the gram-negative rod bacterium that causes the plague was named after its discoverer, Yarsina Pestis.

Kitasato Shibazaburo is a Japanese bacteriologist. His resume is extremely dazzling: under the tutelage of Professor Koch of Germany, who discovered the Mycobacterium tuberculosis, he was the first in the world to isolate tetanus bacillus (1889), and applied serum-free immunity to the treatment of diphtheria and tetanus, opening up the field of serology. After returning to Japan, he founded his own bacteriological research institute and guided Shiga to discover Shigella bacillus (1898), and later founded the Faculty of Medicine at Keio Gijuku, the cradle of today's Japanese prime ministers, and was also awarded a baron.

The most striking stroke on his resume came in 1894, after the third outbreak of the plague, when the plague pathogen Yersinia pestisis was isolated. In the medical community at that time, after conducting preliminary research on the plague bacillus, scientists led by Kitasato Shibazaburo generally recognized that plague was transmitted from rats to humans, and that it was not transmitted from person to person. Therefore, the way to fight the plague is also very simple: exterminate the rodents!

There are four main methods of rodent extermination, one of which is physical equipment rodent extermination: this method has good effect, simple and easy, safe for people and animals, commonly used rat traps, bowl buckles, cylinder traps, etc.; The second is to exterminate rodents manually: exterminate rats in warehouses and houses with intact walls, doors and windows, open the door lightly during the activities of rats out of the hole, run to the entrance of the rat's hole, quickly block the way back, hold a torch in one hand, and throw it in the corner of the wall with something that intends to hide it in the other, and when people are hunting, it will hide in it.

The most effective is the use of sulfur fumigation to control rodents, which is used to eliminate rats on roofs and indoor pests. After the sulfur is burned, a large amount of sulfur dioxide gas is produced, which has a spicy and pungent odor, and after the mouse inhales, the throat is edema, spasms, respiratory paralysis, and suffocation to death.

These methods are suitable for indoor and granary rodent extermination, but if they are used for outdoor rodent extermination, the most used method is to use rat poison, oral poison or gastrointestinal poison, which can enter the gastrointestinal tract through the rat mouth and play a role to kill it directly, and can play a huge effect with a complete set of tools such as sticky rat boards, rat traps, and cages.

After several unprecedented extermination operations, the rats also seemed to think that the Normans were not to be messed with, and that Normandy was a somewhat evil place, and they left in droves and ran towards the warmer and more comfortable kingdom of France.