From the Super Mario Wiki
The Dizziness Virus is a lock-on item which takes the form of a Chill virus from the Dr. Mario series with swirly eyes. Upon a successful hit, the Dizziness Virus will cause the affected racer's view to blur and leave behind afterimages. This makes it very difficult for the racer to see where they are going and take turns properly. The Dizziness Virus can be passed to another racer by bumping into them, removing it from the first racer in the process. Racers affected by a Dizziness Virus are indicated by a spiral effect, as well as the virus itself floating above their heads. After a set period of time, the Dizziness Virus disappears.
Unlike human-controlled racers, CPU racers will lose speed upon being hit by a Dizziness Virus.
A similarly functioning item known as the Wheel Virus exists.