Quarantining smallpox is a most expensive luxury, which may possibly retard the progress of this disease, but was never known to check an epidemic of it. Every epidemic of smallpox during the last one hundred years has been checked by vaccination.
Why some diseases do not return
There are certain diseases which you are not likely to have more than once; one attack protects against another. Why and how does one attack of a certain disease protect against another? When a person is taken sick with one of these diseases, the cells of his body immediately begin to make a substance called antitoxin. We learned something about antitoxin when we were studying diphtheria. In diseases like scarlet fever, measles, and smallpox, in which one attack protects against another, the antitoxin that is formed in the body when you are sick stays there for a long time, in some cases as long as you live. While this antitoxin is present in the blood, the cause of the disease cannot live in the body; hence you cannot have the disease again. After some diseases this antitoxin seems to disappear from the blood in a short time; after others, it seems to remain for several years; and after still others it remains as long as you live. After diphtheria it stays in the blood only a short time, so that one may have diphtheria a second time within a few years. Some people have smallpox, measles, or scarlet fever a second time, but with most people these diseases never return.
If we knew how to make the cells of our bodies produce this antitoxin and keep it stored up in the blood all the time, we should never have any of these diseases. But in many cases we do not know how to cause the cells to manufacture this antitoxin. However, in one or two diseases we do know how to persuade them to make the antitoxin, and the one in which we know how to accomplish this best is smallpox. This is just what is done by vaccination.
How vaccination prevents smallpox
The object of vaccination is to put the cells of the body to work making antitoxin. To do this, it is necessary to get some of the toxin into the body. We want to get in just enough to make the cells work, and no more. Therefore we make a very small scratch, and put into it some of the vaccine which contains the toxin of smallpox. It is impossible to have these germs in your body and not be affected by them to some degree. If you did not feel a little sick, the cells would not be making antitoxin, for the thing that makes you sick is what makes the cells go to work. But this sickness is only a matter of a day or two, and after the cells have made the antitoxin, it will stay in your body a long time, longer in some cases than in others.
Some people, after they have once been vaccinated, can never be successfully vaccinated again; neither can such people ever have smallpox. Most people, however, can be successfully vaccinated every five to seven years, and there are a few people who will "take" if vaccinated every year or two. These conditions indicate the length of time that the antitoxin of smallpox will live in the bodies of these different persons. If vaccination, properly performed, does not take, the person is not in a condition at that time to catch smallpox; and if vaccination, properly performed, does take, it is positive evidence that if this person had been exposed to smallpox, he would have taken the disease.
Necessity of repeated vaccination
It is frequently asked, "How long will vaccination protect against smallpox?" You can no more answer this question than you can tell how long the antitoxin will live in the blood of any particular person. The only safe thing to do is to be vaccinated every few years, and if smallpox is present in your community, get vaccinated every year until the vaccination takes. If it takes, it shows that you were in a condition to catch the disease; and if it does not take, you may feel safe from smallpox for a while, at least.