From the Super Mario Wiki
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The Wheel Virus is a lock-on item which takes the form of a Chill virus from the Dr. Mario series that holds a purple steering wheel. Upon a successful hit, the Wheel Virus will float above the targeted racer's head and periodically turn the racer's steering wheel back and forth, messing up the racer's path. The Wheel Virus can be passed to another racer by bumping into them, removing it from the first racer in the process. After a set period of time, the Wheel Virus disappears.
Unlike human-controlled racers, CPU racers will lose speed upon being hit by a Wheel Virus.
A similarly functioning item known as the Dizziness Virus exists.