Do not mistake mere blood for life. We feed upon something far subtler.
However, do not be surprised if, in feeding, you taste blood within your mouth, for blood and life are intricately interwoven.
We are creatures of both matter and spirit, and thus we affect others on both of these levels. Blood is the physical substance of life, and so we are bound ever so slightly to this material expression of our spiritual need.
Do not be misled by the juxtaposition of energy and blood. No cut or puncture is necessary for the transfer to occur. Our ways are far subtler than that. As you draw out life, you draw out a minute amount of blood with the life. Even when there is no physical contact between you and your partner, when the draw is deep enough, this will occur.
For this reason, always take care in who you feed from, for a taint in their blood may carry over to you.
Never feed from the ill or diseased. Never feed from those who have drugs or alcohol in their system unless you wish to partake of their intoxication for yourself. The taints in their bodies will transfer over to you. You will know those who are tainted by the sensations you get when near them. You will have an instinctual aversion to touching or getting close to such people. Likewise, your senses will warn you if you attempt to initiate a draw. Much of your awareness of the taint will occur on an unspoken level, so be attentive to your impressions and intuitions.
Given time and careful observation, you will learn to discern these sensations in order to identify the taint. This knowledge, however, may bring you great grief, for there are few left in this world who are not tainted in one manner or another.