There are several kinds of rot, generally divided into two categories: dry rot and wet rot. There is no cure for either type, only prevention.
Dry rot occurs in situations where there is a lack of ventilation and a limited amount of warmth and moisture. A suspended ground floor space is ideal; spores spread very quickly and infect masonry and plaster as well as timber. Treatment is by the total removal and burning of infected timber, and the rot-proofing of surrounding surfaces with a special solution. This has to extend at least one metre (3 ft) beyond the visibly affected area. Dealing with dry rot should be left to experts: they will ensure that all the affected timber — with its almost invisible spores — will be discovered, and they will guarantee.
Wet rot is less serious and, while causing timber to decay, does not develop in the same way as dry rot. First, the source of the additional moisture — leaking pipes, defective gutters, etc. — should be repaired. Then the defective wood should be cut out, and the adjacent timbers treated with preservatives.
Woodworm is annoying, but not as serious as rot. Eggs laid by a beetle in the wood, turn into pupae and emerge as larvae, or woodworm. The holes you see are mainly where the pupae have burrowed. Under the surface the insect digs deeper and makes a network of passages. Fortunately it is only in neglected cases that the effect is serious.
Softwoods are commonly attacked and the first attack in hardwoods is in their sapwood, which is usually a lighter colour. There are special insecticides which can be painted or sprayed on to wood, or even injected into individual holes. Rotproofing solutions may also be effective against attack.
Application before an attack is better than waiting until the trouble has started.