Chicken faeces coming out looking like water and rats droppings. Help!

I had to sadly euthanize one of my chickens last night. I noticed a couple of days ago that she seemed to be weak when I went into the coop to top up the water. I put a container of water near her and she walked over and then sat down to drink from the water which I thought was a little odd. However I thought that she might be out of sorts and would be better the following day. That has happened in the past where a chicken seems a little unwell but by the following day is totally better. So I didn't realise how serious it was. I went into the coop yesterday and she was lying down.

She stood up to go to the water container. I saw her stand for a few seconds and then she was too weak to stay standing so she sat down. She was so weak that she didn't event fold her wings properly as she sat down and I had to pick her up and reposition her so that she could sit comfortably. As she was sitting there she sort of was slumping on her side. She couldn't even sit up properly - it looked as though she was almost going to roll over on her side.

Because she was struggling to move I brought food and water near to her. And she drank and ate. But she didn't have the strength to hop up.

A couple of times she didn't even finish her water and it just seemed to fall out of her mouth. She was like that all day and I think only moved maybe about 12 inches all day.

I went inside and later came out and realised that the ground looked wet where she had been. There appeared to be what looked like rats droppings coming out of her. And I saw her do another dropping which was basically looked like only water.

I didn't want to put her down but being that sick I knew that it was the kindest thing to do. I didn't want her to continue suffering because I believe she was dieing and I wasn't sure how many more days she might survive in that weakened and probably painful state.

It was very sad to see her go as I raised her from when she was a tiny baby chicken, only a few days old and she was so tiny that she was smaller than the palm of my hand.

However the reason why I asked for help is because I don't know what caused it and I am worried about my other two chickens. I don't want them to get ill. It seemed to be some kind of intestinal thing. I noticed that as she was lieing on the ground that her body almost seemed to be moving like something was possibly eating away at her, possibly something parasitic - but perhaps that could have been her just trying to do a dropping - it was hard to tell. It's scared me quite a bit and has made me really concerned for my other two chickens. What should I do to make sure my other chickens don't get sick? Any suggestions? Please help. Thank you as I am very worried and concerned.