Portkey-sickness is a condition in which travelling by Portkey causes a disagreement between visually perceived movement and the vestibular system's sense of movement. Hysterics and nausea are common symptoms of Portkey-sickness.
After the establishment of the International Statute of Wizarding Secrecy in the 1692, Portkeys were arranged at gathering points all over Britain, in order to transport Hogwarts School of Witchcraft and Wizardry students in and out of the Castle in a regulated manner, in an attempt to avoid non-magical attention. Portkeys, however, proved not to be the ideal solution: up to a third of the student body would miss their time slot, or would be unable to find their Portkeys and fail to arrive, while those who did arrive frequently filled the hospital wing to bursting due to their Portkey-sickness.
- Pottermore (Mentioned only)