Acute effects occur within hours or days of consuming a contaminant. People can suffer acute health effects from almost any contaminant if they are exposed to high levels. In drinking water, microbes, such as bacteria and viruses, are the contaminants most likely to reach a level high enough to cause acute health effects.
Most of us can fight off microbial contaminants the same way our bodies fight off germs, so these acute contaminants generally do not have permanent effects. When high enough levels occur, though, they can make people ill, and can be dangerous or deadly for a person whose immune system is already compromised.