It is determined by the facts of the case and the laws of the jurisdiction in which the accident occurred. In some cases, a driver may be held liable for an accident if they were negligent in failing to take precautions to prevent the illness or if they were aware that they were at risk of experiencing a sudden illness and did not take adequate precautions.
However, if the driver had no reason to anticipate the illness and was not negligent, they may be exempt from liability for the accident. It would ultimately be up to a court to determine liability based on the facts of the case.