2 H2O() H3O+(aq) + OH-(aq)
At room temperature, about 1 in every 10 000 000 water molecules has dissociated by donating a hydrogen atom to another water. Distilled water under these conditions is actually 10-7M H3O+ and 10-7M OH-.
Hydroxide ion is the caustic component of lye, and hydronium ion makes acid solutions corrosive. However, the concentrations in water are extremely low and not at all dangerous. (For an opposing opinion about the toxicity of water.) Distilled water also reacts with atmospheric carbon dioxide according to:
H2O() + CO2(g) H2CO3(aq)
H2CO3(aq) + H2O() H3O+(aq) + HCO3-(aq)
HCO3-(aq) + H2O() H3O+(aq) + CO32-(aq)
which brings the pH of distilled water in an open-air container down to about 5.8. Distilled water can be found in most Supermarkets. It's great for cleaning your stomach and kidneys as it absorbs calcified minerals as it passes through.