Fever! is a magazine story.
James' driver is sneezing and the red engine thinks he should be at home in bed. James' driver explains that it is just hay fever. James is confused until his driver explains that when trees and flowers put out their seeds in warm weather, it can cause people to sneeze and suffer from watering eyes. James thinks the word "hay fever" is silly; he thinks it should be called "tree fever". Whatever it is called, James is glad to be an engine so that he cannot get any sort of fever.
That afternoon, James is in the yard getting ready for his next job. His driver hopes that they will not be passing the trees again and James assures him that they will not have to on the next journey. James couples up to some coal trucks and heads to the hoppers at Vicarstown. Unfortunately, the workmen have not tied the covers on the trucks properly and streams of black coal dust covers James. The driver starts sneezing again and James becomes convinced that his driver has caught a cold, but the driver explains that the coal dust went up his nose. James jokes that his driver now has "coal fever".
James' next job is to take some feed to Farmer Trotter's Pig Farm. At the farm, it is very muddy. Farmer Trotter explains that it has been raining for weeks. James splashes his way through the puddles and now, not only does he have coal dust over his paintwork, he also has muddy wheels. James is not happy at all.
When they leave the farm to go back to Knapford, road and rail run side-by-side for several miles. A big hay wagon is being pulled by Terence along the road. Hundreds of tiny pieces of hay and dust stream out from it. They blow all over James and stick to his wheels. The driver starts sneezing again, this time it is the straw tickling his nose. James says he must have "straw fever" now.
Back at Knapford, the Fat Controller thinks that James looks very mucky and sends him for a wash-down straight away. James is delighted; usually he has to wait until the end of the day for a wash-down and now there will be no more work for him today.
As the washdown workers begin to scrub James down, bubbles start to float into the air. One bubble goes up James' nose and makes him sneeze. James' driver takes the opportunity to tease James and says it serves him right for making fun of him all day. He says he has never heard of anyone getting "bubble fever" before.