The following is an excerpt from Rilla of Ingleside by L. M. Montgomery. The croup they're fighting is diphtheria.
Then, the third night after father and mother went away, Jims suddenly got worse--oh, so much worse--all at once. Susan and I were all alone. Gertrude had been at Lowbridge when the storm began and had never got back. At first we were not much alarmed. Jims has had several bouts of croup and Susan and Morgan and I have always brought him through without much trouble. But it wasn't very long before we were dreadfully alarmed.
'I never saw croup like this before,' said Susan.
As for me, I knew, when it was too late, what kind of croup it was. I knew it was not the ordinary croup--'false croup' as doctors call it-- but the 'true croup'--and I knew that it was a deadly and dangerous thing. And father was away and there was no doctor nearer than Lowbridge--and we could not 'phone and neither horse nor man could get through the drifts that night.
Gallant little Jims put up a good fight for his life. Susan and I tried every remedy we could think of or find in father's books, but he continued to grow worse. It was heart-rending to see and hear him. He gasped so horribly for breath--the poor little soul--and his face turned a dreadful bluish colour and had such an agonized expression, and he kept struggling with his little hands, as if he were appealing to us to help him somehow. I found myself thinking that the boys who had been gassed at the front must have looked like that, and the thought haunted me amid all my dread and misery over Jims. And all the time the fatal membrane in his wee throat grew and thickened and he couldn't get it up.
Oh, I was just wild! I never realized how dear Jims was to me until that moment. And I felt so utterly helpless.
And then Susan gave up. 'We cannot save him! Oh, if your father was here--look at him, the poor little fellow! I know not what to do.'
I looked at Jims and I thought he was dying. Susan was holding him up in his crib to give him a better chance for breath, but it didn't seem as if he could breathe at all. My little war-baby, with his dear ways and sweet roguish face, was choking to death before my very eyes, and I couldn't help him. I threw down the hot poultice I had ready in despair. Of what use was it? Jims was dying, and it was my fault--I hadn't been careful enough!
Just then--at eleven o'clock at night--the door bell rang. Such a ring --it pealed all over the house above the roar of the storm. Susan couldn't go--she dared not lay Jims down--so I rushed downstairs. In the hall I paused just a minute--I was suddenly overcome by an absurd dread. I thought of a weird story Gertrude had told me once. An aunt of hers was alone in a house one night with her sick husband. She heard a knock at the door. And when she went and opened it there was nothing there--nothing that could be seen, at least. But when she opened the door a deadly cold wind blew in and seemed to sweep past her right up the stairs, although it was a calm, warm summer night outside. Immediately she heard a cry. She ran upstairs--and her husband was dead. And she always believed, so Gertrude said, that when she opened that door she let Death in.
It was so ridiculous of me to feel so frightened. But I was distracted and worn out, and I simply felt for a moment that I dared not open the door--that death was waiting outside. Then I remembered that I had no time to waste--must not be so foolish--I sprang forward and opened the door.
Certainly a cold wind did blow in and filled the hall with a whirl of snow. But there on the threshold stood a form of flesh and blood--Mary Vance, coated from head to foot with snow--and she brought Life, not Death, with her, though I didn't know that then. I just stared at her.
'I haven't been turned out,' grinned Mary, as she stepped in and shut the door. 'I came up to Carter Flagg's two days ago and I've been stormed-stayed there ever since. But old Abbie Flagg got on my nerves at last, and tonight I just made up my mind to come up here. I thought I could wade this far, but I can tell you it was as much as a bargain. Once I thought I was stuck for keeps. Ain't it an awful night?'
I came to myself and knew I must hurry upstairs. I explained as quickly as I could to Mary, and left her trying to brush the snow off. Upstairs I found that Jims was over that paroxysm, but almost as soon as I got back to the room he was in the grip of another. I couldn't do anything but moan and cry--oh, how ashamed I am when I think of it; and yet what could I do--we had tried everything we knew--and then all at once I heard Mary Vance saying loudly behind me, 'Why, that child is dying!'
I whirled around. Didn't I know he was dying--my little Jims! I could have thrown Mary Vance out of the door or the window--anywhere--at that moment. There she stood, cool and composed, looking down at my baby, with those, weird white eyes of hers, as she might look at a choking kitten. I had always disliked Mary Vance--and just then I hated her.
You can read the book to find out what happened to Jims. Wikipedia has more information on the disease, including additional complications, which vaccine prevents it, and the trivia that "diphtheria was cited in the Guinness Book of World Records as 'most resurgent disease.'"