In the 1880s, Louis Pasteur developed a method of weakening viruses. The weakened viruses could be injected into healthy individuals. How is this a method effective in fighting viral diseases?

By injecting dead cells of the disease into the body, the body can analysis what it is supposed to be fighting off before it gets the active diseases. So if the body does actually get the disease then it knows how to fight it off and can have a better chance at doing it.