If you are susceptible to something, such as a disease or emotion, you are likely or inclined to be affected by it.
Sustenance is that which supports life; it usually refers to food or nourishment of some kind or to one's livelihood.
Something sustainable can be maintained or supported over a long period of time.
If something, such as food or love, sustains you, it supports you or keeps you going over a long period of time.
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