Certain parasites are able to enter the youngest parts of trees, twigs and leaves directly, but the majority of the fungi causing decay of the wood can get into the interior of the living tree only by way of a pin knot or wound. For this reason, every wound caused by lightning, by fire, by man, or by animals, constitutes a menace to infection. Many coniferous trees cover their wounds by an aseptic coat of pitch, which is very effective in preventing the germination and growth of fungus spores. But the less resinous conifers and the hardwood trees do not cover their wounds very effectively; large wounds are not covered at all. Upon exposure by a wound, the sapwood just underneath the bark dies, dries out, and checks. Spores of parasitic fungi enter the cracks, germinate and infect the heartwood. The spores of a heartwood-inhabiting fungus cannot germinate and thrive unless they fall upon the heartwood of the tree. In this way certain diseases of the heartwood, which result in rot or decay, can very frequently be traced directly to fire scars, lightning scars, spike tops, broken limbs or branches, and other mechanical destruction caused by lightning, fire, storms, cloudbursts, or heavy snowfall. Fire as a cause of wounds is responsible for more cases of heartrot than all other injuries taken together. For this reason the protection of forests from fire is the most important preventive measure that can be taken to eradicate tree diseases. In fact, the best way of controlling diseases is by preventing them, and the Forest officers are endeavoring to eliminate any danger to the health of the forest, to prevent the injury of the trees, and to establish healthy conditions for their growth. This is forest hygiene, and it bears the same relation to the trees and forests as personal hygiene and community sanitation do to persons and communities.