@Mor - We had the same problem one year with some of our calves. I was quite young and can't remember exactly what was wrong with them, but they were sick and became quite dehydrated.
I remember when the vet came to look at them, he injected some saline and some medicine into their haunches.
He told me to rub firmly at the place where he had injected, in order to spread the medicine. It felt like there was a bubble under the skin and it took what seemed like a while to go away, but was probably only a few minutes.
I was amazed at how quickly the calves seemed to feel better.
Afterwards I wanted to be a vet for a long time, just because he seemed like such a miracle maker.