thus did he observe , categorize , predict , and explain various forms of infection , rot , blight , canker , taint , poisoning , infection , putrefaction , festering , bloating , dysfunction , collapse , decomposition , decay , trauma , fracture , pestilence , rigor , gangrene , parasitism , boil , abscess , tumor , monstrosity , pain , convulsion , depression , defect , disability , decline , and death .