People keep wanting to help them. Often people want to give them water in the summer, but in many cases, that can kill them. They usually go into a rest period in the summer, coinciding with the lack of water for the region. In their rest spell, it is too dry to support many disease organisms (bacteria and fungi), so the plant puts their disease fighting mechanisms to sleep. If somebody waters the plant in its dry resting spell, the bacteria and fungi spring to life, but not the disease fighting mechanisms. This can spell doom for a cactus.