The valves of the heart are likely to give way, partly because they are under such constant strain, snapping backward and forward day and night; and partly, because, in order to be thin enough and strong enough for this kind of work, they have become turned, almost entirely, into stringy, half-dead, fibrous tissue, which has neither the vitality nor the resisting power of the live body-stuffs like muscles, gland-cells, and nerves.

They are so tough, however, that they seldom give way under ordinary wear and tear, as the leather of a pump valve, or of your shoes, might; but the thing which damages them, nine times out of ten, is the germs or poisons of some infectious disease.
These poisons circulating through the blood, sometimes set up a severe inflammation in the valves and the lining of the heart.

Ulcers, or little wart-like growths, form on the valves; and these may either eat away and destroy entirely parts of the valves or, when they heal, leave scars which shorten and twist the valves out of shape, so that they can no longer close the openings.

When this has happened, the heart is in the condition of a pump which will not hold water, because the leather valve in its bucket is broken or warped; and we say that the patient has _valvular_ or _organic_ heart disease.
The disease which most frequently causes this serious defect is rheumatism, or rheumatic fever; but it may also occur after pneumonia, typhoid, blood poisoning, or even after a common cold, or an attack of the grip.

This is one of several reasons why we should endeavor, in every way, to avoid and stop the spread of these infectious diseases; not only are they dangerous in themselves, but although only two of them, rheumatism and pneumonia, frequently attack the heart, all of them do so occasionally, and together they cause nearly nine-tenths of all cases of organic heart disease.
Should you be unfortunate enough to catch one of these diseases, the best preventive against its attacking the heart, or causing serious damage, if it does, is a very simple one--rest in bed until the fever is all gone and your doctor says it is perfectly safe for you to get up; and avoid any severe muscular strain for several months afterward.
This is a most important thing to remember _after all infections and fevers_, no matter how mild.