When we adopt or pick a cat up from the streets, we need to get it used to traveling with us. To do this, it is better if we have a cage or a special bag for them to take in our trips, visits, ...
This is a plastic cage. It is easy to clean.
If we will be carrying it (as well behaved as our cat may be), it can get scared easily and we might even loose it. A scared cat won't heed our calling.
This is another type of bag which is comfortable for cats.
It is better if they get used to it from an early age. And we shouldn't use it just to visit the veterinary clinic. They might turn aggressive when they are in it and to any stranger that might get near.