One of the conditions for an event (which caused the injury/death) to be considered a work accident is that it would be caused by an external cause. 

There is no catalogue of "external causes". Such a cause can be basically any factor causing the accident which led to injury or death and which originates outside the employee's body.

Therefore, we cannot talk about an accident when the incident was caused solely by the employee's natural disorder. An accident, on the other hand, occurs when the incident was a combination of several causes, but the external cause played a decisive role. 

A situation in which an employee suffers epilepsy, a stroke or a heart attack is, as a rule, not an accident at work. However, a heart attack or stroke may be considered a work accident if it was caused by a factor outside the employee's body related to the work performed. 

More in the article by attorneys Piotr Lewandowski and Agnieszka Piasecka "When the cause of a work accident is external"  for Dziennik Gazeta Prawna (article from the 28th of April 2021).