Not getting dewormed could sure play a big part in the immune system. Young animals on a whole just don't have the immune system older animals do. And of course, neither do the older animals. Parasites attack the weaker animals and carrying a worm load could sure make this horse more suspectible to parasites.
Parasites are worms, ticks, lice, fleas, etc. as I am sure you know. If you really look at your cattle, say in the summer, the ones that are not totally shed off are the ones that most likely will have the heaviest fly load. I didn't realize this until it was shown to me. The cattle that don't shed completely off seem to not have the immune system as one that has shed totally. It is very interesting what you find when you are taught to really look.
We had a two-year old in 1994 who was in good shape but was carrying quite a fly load compared to the rest of the cows. The Vigortone area manager was here and he and I were going through the cows. He asked what was wrong with her. "Nothing", I said. He said, "sure there is, look at her hair coat." She hadn't really slicked off, but she was fat. "I said look at her body condition." He came back with, "look at her hair coat." So when we got to the house I looked her up. She had had twins at calving time. I told him this and he said she got hurt a bit when calving. That fall she was open.
It is just things like this I have learned to pay attention to. I enjoy looking at cattle but know I will never get as good as he on observing cattle. He quizes me all the time and I am better at it, but still miss some things. It is fun to learn what the cattle tell me by how they look and act. I always paid attention to the horses and I like cattle a lot. Just didn't know to look for certain tell-tale signs.