There are germs, however, like typhoid fever and diphtheria, which do not produce any particular local disturbance with the growth of bacteria, but the whole body becomes sick, the circulation of the blood is affected, and a general disturbance ensues. This is due to the action of a poison, called a toxin, which is set free as a result of the growth of the bacteria in some one part of the body, which poison is then carried by the blood throughout the entire system, inducing fever and a general debility. Just how these toxins are formed is not certain. They are not the bacteria themselves. This we know because the disease-producing bacteria can be grown in broth and the mixture can be strained through fine porcelain, fine enough to strain out the bacteria. Yet it has been found that the clear liquid passing the porcelain filter is capable of producing disease and is a deadly poison without the presence of any bacteria at all. During the incubation period of a disease, as, for example, in the three-week period when typhoid fever is developing, these poisons are being formed and are being scattered through the body, and it is during this time that the fight takes place between these poisonous forces and the defending forces always present in the human system. As already pointed out, these defensive forces are powerful or not, according as the general health of the individual is good or bad, and we see the familiar sight of persons said to be run down taking a disease, while those not so depleted of vitality are able to resist or remain immune. So certain are scientific men of this power and of the fact that the power resides generally in the white corpuscles of the blood that, in the presence of a dangerous infection, a person's blood may be examined, and, if the white corpuscles are not present in sufficient quantity, proper means must be taken for developing this element in the blood, or else the person must take himself away from the infection, if the infection is to be avoided.