资料题：This passage mainly discuses（　）。

Animals get sick just like people.When they do, they need a doctor.There are doctors that just take care of animals.They work hard to keep them well.Sometimes sick animals can make people sick.So animal doctors help us too when they care for sick animals.Some animal doctors work in cities.They mostly take care of pets.They help a dog that has a cold.They take care of a kitten's eyes.They give dogs and cats shots to keep them from getting sick.Many of these doctors look after birds and rabbits, too.Some animal doctors work in the country.They go from farm to farm.They care for farm animals like pigs and horses.Or they may help a mother cow.These animal doctors put in a long day.Sometimes they must go far out into the country to help a sick farm animal.A zoo animal may need doctors, too.So there are those who work in zoos.These animal doctors must know how to care for all kinds of animals.Sometimes it takes a Lot of time just getting close to a zoo animal to care for it.Let's says lion is sick.How do doctors get near it? First a helper puts food in a small cage.Then the door is opened.The lion goes after the food, and the cage door is closed.Now that the lion is in the small cage, the doctors can get close to it.