Moisture is the hygroscopic or gravitational presence of moisture. Moisture leads to unhygienic conditions besides reducing the strength of structural components of the building.

Moisture causes the following harmful effects.

Moisture is responsible for the breeding of mosquitoes and creates unhealthy living conditions.
Due to dampness, moisture travels through walls and roofs and forms unsightly patches and affects the aesthetics of the building.
In the case of lime plaster, the travel of moisture causes the plaster to soften and collapse.
Swelling occurs due to moisture in the wall and sometimes moisture is responsible for dissolution of bricks, stones, tiles etc.
Wall decorations (like paintings) get damaged due to moisture.
When moisture enters through the floor, the floor loosens due to a decrease in adhesion.
Wood fittings (e.g. doors, windows, almirahs) when in contact with damp walls or floors, tend to wear out, causing the wood to bend, buckle, dry out, etc.
Moisture in a building can be very dangerous if any type of electrical fitting comes into contact with it.
Floor coverings are damaged. Therefore no floor coverings should be used on damp floors.
Moisture promotes and accelerates the growth of termites.
Moisture causes germs of dangerous diseases like tuberculosis, neuralgia, arthritis etc.
Metal fittings tend to rust due to moisture.

Reference

Dr. B.C. Building construction by Punima