Again, not necessarily. In some cases, drinking contaminated water will produce immediate symptoms, like stomach cramps, diarrhea and nausea. In other cases, such as lead poisoning, the toxin builds up in the body over time, and ill effects can take months or years to become apparent. This is particularly true with pregnant women, where lead can cause birth defects, and young children, who can suffer from developmental problems such as stunted growth and lower IQ.
Hardness is not a health concern, but it can have other undesirable consequences, such as preventing soap from lathering, scaling pots and pans, and damaging water heaters.