[please help correct] I've learnt responsibility through keeping a cat

I have a cat named Heibao. He is 16-months old now. I adopted him from a charity girl a year ago. He is very pretty and naughty and is knows how to make me smike. However, several days ago, I noticed there was a tiny bald area on his left ear. I didn't pay much attention until it became red and had a small scab. I was worried so I took him to the animal hospital this afternoon. The doctor examined it very carefully and said it was "microsporum canis", a kind of skin disease which can be very dangerous when left alone and can be passed on to humans. I was so scared. I don't know why a house cat would have this disease. The doctor said it might be the hygienic problem. Sometimes an unclean litter box would trigger the disease. I'm so remorseful because I didn't clean its litter box and scrub its bed duly recently. I felt so sorry about my cat. The doctor prescribed an ointment, and told me it would have a great possibility that Heibao will be ok in a week if I apply the medicine on time. I thanked the doctor and brought Heibao back home. I can't help blaming myself for my laziness with cleaning. It really taught me a lesson about responsibility, like mother, like daughter.