The fact that such individuals are _known to have had the disease_ is a guide for control. The methods to be used are essentially the same as for the sick, (_a_), though obviously such human carriers are much more difficult to deal with since they are well. (_c_) Another class of carriers is those who have never had the disease. Such individuals are common and are very dangerous sources of infection. Many of them have _associated with the sick or with convalescents_ and these should always be suspected of harboring the organisms. Their control differs in no way from that of class (_b_). Unfortunately a history of such association is too often not available. Modern transportation and modern social habits are largely responsible for the nearly universal distribution of this type of carrier. Their detection is probably the largest single problem in the prevention of infectious diseases. A partial solution would be universal bacteriological examination. In our present stage of progress this is impossible and would not detect carriers of diseases of unknown cause. The various classes of carriers just discussed are in a large part responsible for the continued presence of the commoner diseases throughout the country. The difficulties in control have been mentioned. A complete solution of the problem is not yet obtained. The army experience of the past few years in the control of infectious diseases shows what may be done. There is another class of carriers which might be called the “universal carrier,” _i.e._, there are certain organisms which seem to be constantly or almost constantly present in or on the human body. These are _micrococci_, _streptococci_ and _pneumococci_, all _Gram positive_ organisms. They are ordinarily harmless parasites, but on occasion may give rise to serious, even fatal, infection. Infected wounds, pimples, boils, “common colds,” most “sore throats,” bronchitis, pneumonia are pathological conditions that come in this class. Such infections are usually autogenous. There is a constant interchange of these organisms among individuals closely associated, so that all of a group usually harbor the same type though no one individual can be called _the_ carrier. Whenever, for any reason, the resistance of an individual (see Chaps. XXV et seq.) is lowered either locally or generally some of these organisms are liable to gain a foothold and cause infection. It sometimes happens that a strain of dangerous organisms may be developed in an individual in this way which is passed around to others with its virulence increased and thus cause an epidemic. Or, since all of the group are living under the same conditions the resistance of all or many of them may be lowered from the same general cause and an epidemic result from the organism common to all (pneumonia after measles, scarlet fever and influenza in camps). Protection of the individual is chiefly a personal question, _i.e._, by keeping up the “normal healthy