It began in 1865 when his old friend, Professor Dumas, asked him to go to the south of France, Alais where he lived to investigate the epidemic that was killing the silkworms. It was devastating the famous silk industry, bringing ruin to the people there.
Louis was reluctant to go but Dumas was an old friend so Louis set off with one of his best pupils, Duclaux, with three other students from the Ecole Normale. Madame Pasteur, was prepared to turn her hand to whatever her extraordinary husband might do. And of course the family went too. In the cottage in Alais they settled down to solve the tragic mystery caused by the unknown silkworm disease.
The disease seemed to start on the surface of the silkworms, like a dusting of pepper-grains. Here in the south of France it was called pebrine, from the local name for pepper.
Hundreds of silk chrysalises and moths went under Louis microscope. Within a few days he decided that a little globule seen in diseased worms, moths and chrysalises was a sure sign of the disease. He decided that it started with mature moths producing diseased eggs which developed into diseased worms, moths and chrysalises.
He got the solution at last. Check the moth after she had laid eggs, for any globules in her body. If there are, then the eggs would be diseased and must be destroyed. If the moth's body is clear of globules, the eggs would be sound, and healthy worms would emerge.
He wanted to try out his theory but he had to wait till the eggs hatched in the spring to find out if his predictions were right. Louis learned a bitter lesson. Spring came; the eggs he thought came from healthy moths produced diseased worms; hundreds of silkworm breeders, relying on his method of sorting eggs, faced disaster.
He had made a dreadful mistake. He went back to his microscope to find where the mistake was. He struggled and he tried experiments that didn't work, and he couldn't understand why: there were dying worms which didn't have globules, and live worms that did. He felt terribly responsible, and suffered bitter attacks from the breeders and his enemies.
But his pupils were not discouraged. They went on with the experiments he asked them to do, and the work continued, month after month.
It was an epic of scientific investigation and struggle. In the end, they found where the mistake had been: there were two diseases, not one, one with globules, and one with another microscopic creature, quite different. They had learned a vital fact about the pebrine globule: it was alive. It was a microbe. It multiplied, spreading throughout the moth, egg or worm.
So Louis saved the people of the silkworm lands who depended on these little creatures spinning their silken cocoons. He learned how vital it was to be unfailingly methodical and complete in his work. He had found out also that healthy worms became sick when the droppings from sick worms soiled the mulberry leaves they ate: the second disease that had confused him, flacherie, was passed on through the worms intestines. In effect he showed the importance of the environment in spreading disease.