Little by little the disease gained headway, until by the time its true character was understood it had taken a hold on the people and had become difficult to stamp out. The strictest quarantine regulations were enforced as soon as the sickness was proved to be true yellow fever, even the passengers on the trains being inspected and closely watched before they were allowed to pass from infected districts to those which were free from the dreaded disease. With all the care it continued to increase, and has not yet been controlled. On such occasions the scientists are always very busy. While some of the doctors are trying to cure the disease, others are busy preventing the sick persons from carrying the contagion to other places, and others again are occupied in trying to find the cause of the epidemic, and how to prevent it in future. One of the scientists who have been working to prevent the disease has discovered the microbe which causes yellow fever, and claims that an epidemic can in future be prevented by inoculating people with it in the same way that they are now vaccinated for small-pox. Small-pox was at one time a scourge throughout the world, and fearful outbreaks of this plague would occur wherever numbers of people were gathered together. About the year 1718 an English lady travelling in Turkey noticed that inoculation was practised in that country with the greatest success, and that epidemics were greatly prevented thereby. This lady, Lady Mary Wortley Montagu, introduced the practice into England. The idea was to introduce into the blood the germs of the dreaded disease, practically giving the patient a slight attack of small-pox, which made him proof against another attack. Inoculation was, however, objected to, because sometimes the person operated on took the disease in its violent form, and died from the results. The fact, however, remained that people who had been inoculated were not liable to take the disease again, and so much good resulted that the physicians were constantly seeking a means of inoculating that would insure only a mild form of the disease. The problem was at last solved by the great English physician, Edward Jenner, in 1798. He found that a form of small-pox was prevalent among cows, and that by taking the germs of this disease, which was called cow-pox, and putting them into the blood of human beings, he could produce a mild form of small-pox, which never assumed a dangerous character, and yet prevented the person treated from taking the real deadly small-pox. From this experiment vaccination, as we know it to-day, resulted. The practice was given this name in France; the word is derived from _vacca_, the Latin for cow. Since vaccination became general, the decrease in the rate of deaths from small-pox has been wonderful, and there has not been one serious epidemic where the practice has been followed.