Letters 85.12

If the existence of the passions is not in our own control, neither is the extent of their power; for if you once permit them to get a start, they will increase along with their causes, and they will be of whatever extent they shall grow to be. Moreover, no matter how small these vices are, they grow greater. That which is harmful never keeps within bounds. No matter how trifling diseases are at the beginning, they creep on apace; and sometimes the slightest augmentation of disease lays low the enfeebled body!

Seneca, Letters to Lucilius 85.12, translated by Richard Mott Gummere