Zelenka was born in 1976 and worked as a nurse on an internal medicine ward at the hospital in Havlickuv Brod, in the Czech Republic.
Between May and September 2006 he gave patients large unprescribed doses of heparin, an anticoagulant, which caused uncontrollable internal bleeding. Seven patients died and ten more suffered serious harm and survived. The victims were men and women, most of them elderly and already ill, and the deaths appeared at first to be complications of their conditions. Czech reporting called him the Heparin Killer, after the anticoagulant he used.
He was identified when a doctor noticed the number of unexplained bleeding events on one ward and on one shift pattern, and hospital records were audited. He was arrested on December 1, 2006.
He was tried at Hradec Kralove and sentenced to life imprisonment on February 22, 2008. He gave several accounts, at various times denying the deaths, describing an interest in seeing whether the resuscitations would work, and describing a grudge against colleagues; no motive was settled on by the court.
The case led to changes in the control of anticoagulants in Czech hospitals. It belongs with Marianne Nolle in this archive as a case where the drug used was one the ward already had in quantity, needed no explaining, and produced deaths that looked like the illness the patient came in with.