The reason this happens is because there has been an exchange of energy between two people. Everything is energy and we are no different. In the same way that we can catch a virus we can also be affected by positive and negative energy being emitted from a person. If we do not recognise this and learn how to deal with it, this negative energy can build up in us and cause feelings of lethargy and even depression or illness.
So how do we prevent this? Of course prevention is better than cure. There are ways we deal with negative energy being emitted. When I go into a paranormal investigation I put up and reinforce my protection. This is my protective energy field around my aura. I simply imagine and white light coming from the universe above and surrounding me from head to toe and sealed at my feet. This provides protection from any negative energies/entities I may encounter. So when you are in the company of a person who is draining you and affecting your energy with negativity, just simply imagine the white light surrounding you and shielding you from their energy, give it a try and let me know how you get on.. it really is effective!
If, however, you have already been affected by negative energy and need a spiritual MOT, then that needs to be performed professionally and will clear negative energy from your Chakras. It is a "combination spiritual" treatment of Chakra Cleansing Reiki & Meditation. After receiving a treatment like this you will feel lighter, refreshed, clear headed, healthier and happier.
So we need to take care of our physical health as well as our spiritual health, the two go hand in hand.