Fervor and fever are easily confused words.
The spell-check application of most word processing software programs would not catch a slip-up of these two words. Spell-check is looking for words that aren’t in its dictionary, and words that resemble words in its dictionary but are possibly spelled wrong. Spell-check isn’t perfect. It doesn’t know and can’t guess what word you wanted or what word you meant, it can only judge the words on the page. If you used words that are all spelled correctly, it gives you a pass anyway.
Autocorrect suggests words that start with the same letters. It’s suggesting what word you may want to save time, but quite often, its suggestions are pretty off base. They don’t help you out, but they do make you laugh.
Fervor is a noun. It means exhibiting energy, passion, and enthusiasm when speaking, dancing, etc. It can also mean intense heat.
Fever is a noun. It means when the body’s temperature rises to fight off infection. Typically fever is measured by feeling a person’s forehead for warmth, or using a thermometer in the mouth, ear, or with babies, the anus.
Fever can also be used figuratively. If someone catches a fever for an activity, it means that person has discovered a new hobby and can’t get enough of it.
The following story uses both words correctly:
Fern didn’t know what to expect from her first Zumba class, but the music really motivated her. She performed the routines clumsily, but with fervor. Clearly, she had caught dance fever. She immediately enrolled in ballet to develop more grace in her steps.