And if Christ [be] in you, the body [is] dead because of sin; but the Spirit [is] life because of righteousness.
Note 23 at Ro 8:10: Based on Ro 8:9, as well as Joh 14:20, 2Co 13:5, and Col 1:27, Christ is in every born-again believer. Therefore, Paul was saying that for all Christians, our bodies are dead because of sin (see note 24 at this verse).
Note 24 at Ro 8:10: The body is dead "because" of sin, but the Spirit is life "because" of righteousness. The Greek word translated "because" here is "DIA," and it denotes "the channel of an act; through" (Strong's Concordance). Therefore, our bodies are dead through, or because of, the influence of sin in our lives.
In the same way that some people who have recovered from the polio virus still have crippled bodies, so Christians who have been delivered from the old sin nature (see note 9 at Ro 5:21) still have to deal with the corruption that the old sin nature released into our physical bodies and minds. That's why no one can please God in the flesh. The flesh has been corrupted and is therefore dead, or incapable of living up to God's standard.
To counter this, the Spirit of God is releasing life because of our new righteous spirits that we received through faith in Jesus. Sin has left its mark on our bodies, but the Spirit of life within us is more than enough to overcome these problems. That's why it is imperative that we Christians learn how to walk after the Spirit and not after the flesh.