There are many different kinds of acids. The proteins in our food, and in our bodies, are made up of amino acids.
Motor cars start because of the sulphuric acid in their batteries.
They are not harmful and are used in preparing our food. The only place on earth where pure water is found is in a laboratory. Rain water always contains small amounts of impurities. These impurities come from dust particles or are absorbed from the gases in the air. If pure water is exposed to the air it absorbs carbon dioxide to form carbonic acid and becomes slightly acidic, dropping from pH 7 i.e. neutral, to pH 5,6.