When the creator created the man, He made the flesh out of the dust of the ground and breath the breath of life into it and man came alife.
When the man came alife, two beings came alife in him. The first being is the Spirit of God while the other being is the Soul.
The Soul is the mind of Adam and he is a being different from God's Spirit given to him. The Soul has it's mind while God's Spirit has his. Anything that has a mind is alife. A man dies the very time his mind stop working.
The forbidden fruit was eatten to profit the human flesh who does not have a mind at creation. The effect activated the human flesh to life by given him a life of evil.
Now the human flesh becomes alife to evil and not to good. Anytime God's Spirit in us teaches us to do good the mind of our flesh teaches us to disobey and do evil. The man now has the mind of the Spirit of God, his soul mind and the mind of the flesh.
Because God know the mind of the flesh is evil, He warn us against him. He that works according to the desires of :the flesh shall surely die but he that works according to the desires of God's Spirit shall live eternal life.
Anything that has desire, has life because your desire is to future or present want and it makes you live for something. When we don't have any desire in life that life means nothing to us and we would love to end it.
The flesh is sinful because he is the embodiment of all evil if it is not checkmate. The evil forbidden fruit was eatten inside the flesh and not the Soul or God's Spirit because they don't eat fruits.
God's Spirit is good, infact He is with Adam for the purpose of feeding Adam's soul with all goof things. Adam already knows good through the Spirit of God in him before eatten the forbidden. The forbidden fruit only made him to know what was evil.
The evil (sinful nature) was conceived inside the flesh.