Demons are messengers of hell sent to trouble us (2 Corinthians 12:7). Witches, wizards and demons are the gates of hell contending against our glory.  

Demons gain access into people's blood before they can totally destroy them. They come to steal, kill and destroy. 

The blood of man is the fluid of life. It supplies life (oxygen) to the body. In the psychological and spiritual aspect, man's soul which is his achievements or history supplies light to his destiny (career or calling). This is why we often dream of our past environments. That's our soul. With it we can see the mysteries of our lives. Mysteries are prophetic.

In Genesis 4 verse 10,  God says "the voice of your brother’s blood cries out to Me from the ground..."    God was referring to Abel's past prior to his death. 

When demon access the blood of a person, they intend to pollute it. There are ways demons can have access to ones blood:

Ways Demons Can Have Access To Ones Blood

According to Paul, Satan can appear as an angel of light in order to seduce man (2 Corinthians 11:14). 

Through seduction Satan gains your trust, and through your trust he gets into your blood.

We are seduced because of our love for gains, not for God. When you love God, you can't give into seduction. But when you love money, fame, power, and sex, falling into the darkness of seduction is inevitable. Through seduction Satan can destroy ones achievements. Beware: not all that glitters is gold. Not everything thing is good for you.  

2.  Fornication:  Fornication is a huge platform where demons can get a complete hold over ones life. Fornication is a soul destroying activity. Demons exploit the root of a man through fornication. Fornication can be physical. Fornication can be spiritual. Spiritual fornication is carried out while men slept. Witches and wizard can have sex with someones spirit while they are asleep. Be careful of the kind of people deal with in your waking life. 

3. Eating Food Sacrificed to Idols: Witches and wizards can give you food offered to demonic idols in their coven. When you eats such food, they use it to gain access to your blood. Also one can eat from the table of demons in dreams which can affect their life physically.