Lead.
"Due to insufficient water treatment, lead leached from water pipes into the drinking water, exposing over 100,000 residents to elevated lead levels."
The cause of death of 12 people by Flint's water is lead and possibly Legionella bacteria.
It is a gram negative bacteria to cause a disease called legionellosis, it is a pneumonia like disease.
They are present in water and people get infected by these bacteria when they drink the water.
Thus, the correct options are lead and Legionella bacteria.
Learn more about Legionella bacteria
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