At the hospital the doctors couldn’t figure out how this had happened until they tested all the longest of long shots. It appears that when Steve gave his grandmother a betta fish for Mother’s Day he thought her had given her just the right gift. But what he had actually given her was a repository of rare Thai fish bacteria, Mycobacterium Marinum.
The bacteria, getting into her finger through a small wound while she cleaned the fish bowl had quickly infected her finger. Unfortunately, the hospital had no capacity to deal with rare Thai fish bacteria and in any case, it was too late; the doctors amputated her finger.
Steve could never look upon her again, waving her four remaining fingers at them from the porch as they drove away, without feeling a twinge of guilt.