How did Thomas Stonewall Jackson catch pneumonia?

Thomas "Stonewall" Jackson caught pneumonia after being accidentally shot by his own men during the Battle of Chancellorsville in May 1863. Following the injury, he was transported to a field hospital where he developed pneumonia, likely due to exposure to the elements and the conditions of the makeshift medical facility. Despite the efforts to treat him, he succumbed to the illness a week later, on May 10, 1863.