After Adam fell by sin and sank into corruption, at once impure pleasures rushed in, and the law of the jungle sprang up in our members. So nature became sick with sin through the disobedience of one, Adam. Then the many became sinners, not as fellow transgressors with Adam, for they did not even exist, but as being of that nature which had fallen under the law of sin. Human nature in Adam became sick through the corruption of disobedience, and thus the passions entered into it.
Death came to the whole human race by Adam, just as when the root of a plant is injured, all the young shoots that come from it must wither.
Based on Saint Cyril of Alexandria
— Source: asceticexperience.com