Mould and mildew appear in some damp conditions. It is necessary to remove these infestations as part of the preparation for painting.
Mould is a fungus. It will grow in warm moist conditions that has some (organic) food source. When mould grows in a house it steadily damages the wall. It cannot simply be painted over.
Mould is a health risk. Breathing in the spores can cause lethargy, cognitive problems and respiratory problems.
Mildew is a milder form of mould, only appearing on the surface of buildings, and not posing any health risk. It does smell rather bad, and it must be removed for painting.
Heavy mould will require professional treatment; it is difficult to remove and many have caused serious damage.
Small mould infestations can be removed with appropriate cleaning agents. These specialized mould and mildew cleaning liquids are available from hardware stores. They often need to be diluted in water before spraying on the mould inflected area.
Alternately, small mould infestations can be removed with diluted bleach; mix 1 part bleach with 2 or 1 part water.
Often the area affected by mould will need more than 1 treatment.
The solutions used to clean mould are quite strong, and will causing fading to fabrics. Take precautions and keep away from carpets, rugs and clothing.
Removal of mould and proper preparations of all wall surfaces help produce the best painting results