_Bread_ forms an excellent medium for moulds, but unless specially exposed the bacteria in it are few. Waldo and Walsh have, however, demonstrated that baking does not sterilise the interior of bread. These observers cultivated numerous bacteria from the centre of newly baked London loaves.[76] The writer has recently made a series of examinations of the air of several underground bakehouses in Central London; but, though the air was highly impregnated with flour-dust, few bacteria were present. Other foods and beverages may be, and are, from time to time contaminated in some small degree with bacteria or their spores. Such contaminations are generally due to uncleanly manufacture or unprotected storage. The principles of examination or of the prevention of pollution are similar to those already described. CHAPTER VII _THE QUESTION OF IMMUNITY AND ANTITOXINS_ The term _natural immunity_ is used to denote natural resistance to some particular specific disease. It may refer to race, or age, or individual idiosyncrasies. We not infrequently meet with examples of this freedom from disease. Certain races of men do not, as a rule, take certain diseases. For example, plague and leprosy, though endemic in some countries, fail to get a footing in England. This, of course, is due in great measure to the sanitary organisation and cleanly customs of the English people. Still, it is also due to the fact that the English appear in some degree to be immune. Some authorities hold that the immunity against leprosy is due to the fact that the disease has exhausted itself in the English race. However that may be, we know that immunity, entire or partial, exists. Children, again, are susceptible to certain diseases and insusceptible to certain others to which older people are susceptible. We know, too, that some individuals have a marked protection against some diseases. Some people coming into the way of infection at once fall victims to the disease, whilst others appear to be proof against it. It is only in recent times that any very intelligent explanations have been offered to account for this phenomenon. The most recent of these, and that which appears to have most to substantiate it, is known as immunity due to _antitoxins_. _The products of bacteria_ are chiefly six: 1. _Pigment._ We have already seen how many organisms exhibit their energy in the formation of many coloured pigments. They are, as a rule, "innocent" microbes. Oxygen is required for some, darkness for others, and they all vary according to the medium upon which they are growing. Red milk, yellow milk, and green pus afford examples of pigment produced by bacteria.