G'day justdoit,
Have certainly done so here. We're absolute victims whenever they get ill. It totally depends on exactly what it is that's making them sick though.
Severe gut infections/colitis x /salmonella scours in young horses are my greatest fear. It comes on like a runaway bus, hangs around for what seems like forever and they will lose more fluid than you can ever manage to put into them via a drip. If they live they never seem to get back to what you'd have reasonably expected them to be. They will almost always seem dull in the coat and end up generally poor doing. I think the toxins produced by the bug/s destroy the normal working of the digestive tract. I think it may even paralyze parts of their gut & to varying degrees it then inhibits their ability to both properly push through and moreso to properly absord the full range of nutrients from their food. It seems to leave them overtly prone to Colic because of that....this being especially prevalent after you worm them in the normal course of things. I'm sure it also stuffs their kidneys by way of severe dehydration and so then, on a related note the poor kidney function causes heart related issues from electrolyte imbalances that you can't correct no matter what you try to do.
Rhesus Factor is a shocker too. It is fairly rare thankfully but if you don't spot it early and get to it quickly I've found that even if they do survive they'll rarely recover fully. Similarly to the colitus x/scours angle, they never really seem to get over it and go on to do well, often ending up runty and poor doing.
Other kinds of infections don't seem to have as many/any lasting issues despite the often horrific visuals that they can give.
Blood born infections like Tetanus etc. are visually pretty bad, mainly because of the awful speed in which they can progress...sometimes you can watch them start to go downhill right before your eyes...but again, with treatment young horses seem to fully recover just as quickly. Some people get bent out of shape with lung infections in young horses, to some degree with good reason, but unless it is really severe and there is really significant scarring, the lungs seem to be more forgiving than the gut or a foal knocked by RH. We also had a weanling filly with the all-time WORST case of strangles you have ever seen and she made a complete recovery & went on to be a very good racemare.
From the standpoint of withstanding various health impositions a lot of people automatically think because they're young that foals are therefore fragile. I don't believe this to be the case at all. You've only got to see what they go through being born to know that they can cope with a lot more than we give them credit for. They're actually quite robust things if given half a chance. They can and do bounce back very quickly at times, moreso and much better than older horses with comparable issues. I think this is because they are in a full on, all out growing phase and they invariably get better much more quickly. It's kind of like breaking your arm or leg. You're much better off doing it when you're 15 than when you're 55.