Poison.

Although it is not as likely to come into contact with poison ivy during the winter months, particullarly where it snows it is still possible to contract "poison ivy" or rather the allergic reaction associated with it during the winter. All it takes is for the sap (urushiol - the component that causes the allergic reaction) that runs all through the the plant to make contact with your skin. If you walk through it even once it has died out, burn it (leaf piles or burning brush) or come into contact with anything this oil may have been previously deposited on you may well end up with a case of poison ivy. You can even get it from your dog if you pet him and he previously ran through it. I’ve had it numerous times and know how miserable it can be and how easily it can spread.
Poison ivy identification is often summed up with the maxim, "Leaves of three, let it be!" Also note the notch on one side of each leaf which is also a good identifier of poison ivy.
That’s poison ivy, the "hairs" being the vines’ aerial roots. As you can see in this picture of a poison ivy vine climbing a tree, the vine can cling so tightly to the trunk of a tree as to seem a part of it. It’s only the aerial roots that give the vine away in this picture.