When I got some colts from a well known farm last year, they arrived sick (and one horribly underweight). There was a very slight cough that happened so rarely, a lot of owners would never even have heard it, and "mildly" snotty noses. Our vet was out and we treated them aggressively (and quite expensively). What they had was Rhodo. Pneumonia, which per the vet and the state lab, they got long before they arrived at my place (long incubation and only susceptible at early ages).
These colts, unfortunately, died. However, I learned from dealing with them that foals can be extremely ill yet run around, eat, and act really normal. Additionally, ours did not have any fever at all until the nights before they died, when they spiked -- but this could have been a side effect of one of the antibiotics. Untreated, this illness has an 80% fatality rate so not treating was not an option (at least not with H and I as the owners) and you must use a combo of 2 out of 3 specific antibiotics to kill this disease. Standard antibiotic will not help at all and cost valuable time.
Bottom line, when it comes to foals, do not try to self diagnose or treat w/o a vet. Don't put a lot of stock into a lack of fever or active behavior because this serious illness normally does NOT cause a fever and the foals generally act like they feel well. It's something only a vet can diagnosis. Foals can go down hill so quickly and I would really urge you to have the vet out asap.
