Summary The hookworm index or average number of hookworms harbored by a sample of the population is not only an indicator of the degree of infection in a group of persons, but is an index of the degree of soil infestation and of exposure to it in the district or environment habitually used by the group. This index is a more accurate expression of the amount of hookworm infection in a community than a statement of the percentage of the people positive for hookworm ova; for the Uncinariasis Commission to the Orient found that of two groups of people who had 90 to 100 per cent of their number infected with hookworm, one group had an average of only 35 hookworms per man while the other had 273 worms per man. The anemia and severity of the symptoms in general correspond with the number of hookworms harbored. This may not be apparent in the individual case because individual resistance may cloak the malignant influence of the worms which are none the less causing losses of blood and drains on the individuals powers. Groups of persons and types of communities possess certain characteristic grades of infection. Agriculturists usually have a high worm index, while town-dwellers have a low index. Intimate barefooted contact with infested soil is the factor of greatest importance in raising the hookworm index. Diagnosis by vermicide is more accurate than diagnosis by microscopic examination for ova. It is important to know more about the personal hygiene of people and sites of pollution, for what has been called racial immunity may be nothing more than the result of different degrees of exposure to infested soil following different occupations, etc. The relative immunity of the negro in the South and the apparent higher susceptibility of the white man may be due to the scattering of infection by the necessities of his vocation in the former, and the concentration of infestation in highly polluted nests by the whites. There is some correspondence between the number of ova discharged in feces and the number of female hookworms harbored when large numbers of cases are considered. Recent work by Baermann and Cort and Augustin lead us to believe that embryos of hookworms do not live for a long time in the soil, but in the tropics, at any rate, survive but a few weeks. In hookworm control, it would seem to be strategic to treat by vermicide all the people in a community within as short a period of time as possible so that soil pollution and soil infestation will not continue but be abruptly terminated, and so that there will be no re-infection of those already treated. Mass treatment of entire communities is recommended wherever it is practicable. It is believed that when an entire community is purified of most of its worms by mass treatment within a few days, very few hookworm embryos will be passed to infest the soil, and the latter instead of remaining highly infested, as before treatment, rapidly becomes lightly infested and only light and relatively very harmless infections can result. In places like China where agriculture is only economically possible through the use of dejecta as fertilizer and where hookworm campaigns are not thought to be feasible, it would seem to be possible to rid the soil of hookworm larvae by mass treatment of communities.