If I was able to create an image of negativism, it would look like the snot character you see for the Mucinex cough commercials. It always has its bags packed and ready to follow the next victim it can infect.
Being around someone who carries a high degree of negativism is putting yourself in an infectious zone. The longer you expose yourself to it the better chance you have of catching the germs and spreading it around to others. Like a bad cold you wipe your miserable runny nose with your hands touching everything within your line of path. Your annoying cough and sneezing spreads spores of negativism in the air where they are ready to jump onto the next person who walks within its negative trap. It feeds on itself the more negativism it can collect the bigger it grows, and like wildfire It becomes hard to control.
Your best remedy is to get as far away from it as you can, let it take its course as it slowly works its way out of you. Feed you thoughts with optimistic value, and watch as its antibiotic affect wipes the negativity right out of your system.