What could be wrong with the dog?

One of my dog owner's this morning (I'm a dog walker) rung me to say that her dog has been taken to the vet. She's been kept overnight and has had to have blood tests. Apparently she woke up and the dog didn't want to move and she couldn't stand up.
The day before she had been a walk with me in the morning (nothing out of the ordinary). Then the owner took her out in the evening she said she was running around. She did say in passing that at one point she sprinted off when she saw a fox.
Could it be something, if she came across a fox that could be passed on to her? I know its hard to diagnose. Feeling worried now.
She said she couldn't put one foot in front of the other and her neck was stiff. Couldn't stand. I feel just eating something or stepping on something wouldn't cause this.

 

They are suggesting polyarthritis. They are putting her on steroids.