I have had the misfortune of getting a rash from poison ivy multiple time. Usually, it is a small patch on my hands or arms. It is annoying and uncomfortable, but it goes away in about a week. I am lucky I have never had a serious reaction.
I came across this picture on Reddit and do not know any context about what happened to result in such a serious reaction. All I can say is that the man must have been miserable and hopefully got medical treatment. I can not imagine what that must have felt like.
According to Web MD, the reaction is caused by a chemical called Urushiol. It is present in all parts of the plant. The rash that results from the poison plants is a form of allergic contact dermatitis. An allergic reaction when it comes into contact with skin, resulting in an itchy rash, which can appear within hours of exposure or up to several days later.
From what I understand the severity of the rash can because by the amount of Urushiol a person is exposed to or the sensitivity of the person. So when a super sensitive person is exposed to a lot of Urushiol, this is the result.