The head of the Michigan health department, Nick Lyon, has been charged with involuntary manslaughter in Flint’s lead-tainted water crisis.

Lyon is accused of failing to alert the public about an outbreak of Legionnaires’ disease in the Flint area between 2014 and 2015 which has been linked by some experts to poor water quality.

Legionnaires’ is a severe form of pneumonia caused by a bacterium known as legionella. Although legionnaires’ disease primarily affects the lungs, it occasionally can cause infections in wounds and in other parts of the body, including the heart.

There were roughly 100 cases of Legionnaires’ disease in 2014 and 2015 in the Flint area, including 12 deaths.

Lyon is the highest-ranking official to be charged in the Flint water crisis probe.

Flint began using water from the Flint River in 2014 but officials decided not to add an anti-corrosive agent that had been suggested by a study to make the water drinkable. The agent would have stopped the water from corroding the city’s pipes and avoid lead from old plumbing to leach into the water system.