The silent killer, well probably not if you are reasonably healthy, but mould if left untreated can make you very, very ill and if you already suffer from respiratory problems will make them worse, a lot worse.
The picture shows what happens if you leave damp untreated, the black and grey discolourations are mould clusters, they attract house and dust mites who feed on the mould and also release microscopic spores that permeate the air and enter your lungs when you breathe. Do you ever get a tight chest when being in a room for a while? if you do then chances are there is mould growing somewhere in there. While a hazard to your health as well as being disgusting to look at, it can be treated and eradicated relatively easily and fairly inexpensively too (unless the problem is too advanced in which case a professional removal service would be required and that can be pricey).
The first mould you would probably notice is in the bathroom or kitchen tile grout, little black spots or discolourations against the grout. A simple spray with a fungicidal mould remover like HG Mould Spray and wipe down afterward, will take care of that and prevent it from returning.
For much larger patches on walls or ceilings for instance then it is a bit more involved.
Firstly, you are going to need fungicidal wash, use this to wash down the affected area and surrounding walls after first removing any wallpaper or paint (make sure the room is well ventilated and use a painters mask to avoid inhaling the mould spores when you remove the wallpaper or paint). The wash will kill any mould in the plaster surface and prevent new cultures from growing.
If you are repainting the previously affected area, then you can get specially treated fungicidal paint that prevents the mould from regrowing or for a more inexpensive option, you can get a paint additive that mixes with normal paint for the same result.
for wallpaper, you will need to purchase fungicidal wallpaper paste to use to prevent a recurrence.
Obviously the best course of action is to not allow mould to grow in the first place by controlling the damp in your home where possible, but if you do find yourself with a funny discolouration in the corner of the room or on the window lintel, then hopefully you have gotten to it in time and can make use of one of these anti -mould products before the pennies start to rack up.