A number of examples from recent research are given of the influence of protein and amino-acids on the metabolism of infecting organisms; of infecting organisms on host metabolism; and of the role of protein in determining the susceptibility and resistance to infection and its course when established. A comparatively neglected aspect of the subject of host-parasite relationships is the effect of the zymotic disease factor on the nutritional status of the host. Some success may attend treatment either by dietary means or by eradication of the infecting agent. There are however grounds for believing that for complete recovery a two-pronged attack is likely to be necessary. Also, even if the nutritional defect is predominantly of protein origin, it will no doubt be found that the best results will follow rehabilitation on a diet adequate and balanced in all respects. It is also noted that the repletion of protein-depleted subjects may require many months of protein feeding at levels above that to which the diseased patient had been accustomed.