Colds, influenza and pneumonia have been looked upon as causes of consumption and are still regarded as such. They are not primary causes but secondary causes. When the tubercle bacillus has gotten into the system these diseases can help to break down the individual and hurry up the tuberculous process but they cannot themselves produce tuberculosis. The injury which they themselves can do may also prepare the individual for the tubercle bacillus. They are themselves due to living micro-organisms which are independent entities. Colds are probably due to various kinds of microorganisms whilst influenza and pneumonia are due each to a specific micro-organism. A micro-organism can only re-produce its own kind and can never be transformed into other micro-organisms. A cold will therefore always produce a cold, influenza will always produce influenza, pneumonia will always produce pneumonia and tuberculosis will always produce tuberculosis. For a person who has tuberculosis an attack of cold, influenza or pneumonia is a serious matter and for this reason tuberculous people should keep away from people who have those diseases.