communicable diseases, illnesses caused by microorganisms and transmitted from an infected person or animal to another person or animal. Some diseases are passed on by direct or indirect contact with infected persons or with their excretions. Most diseases are spread through contact or close proximity because the causative bacteria or viruses are airborne; i.e., they can be expelled from the nose and mouth of the infected person and inhaled by anyone in the vicinity. Such diseases include diphtheria, scarlet fever, measles, mumps, whooping cough, influenza, and smallpox. Some infectious diseases can be spread only indirectly, usually through contaminated food or water, e.g., typhoid, cholera, dysentery. Still other infections are introduced into the body by animal or insect carriers, e.g., rabies, malaria, encephalitis, Rocky Mountain spotted fever. The human disease carriers, i.e., the healthy persons who may be immune to the organisms they harbor, are also a source of transmission. Some infective organisms require specific circumstances for their transmission, e.g., sexual contact in syphilis and gonorrhea, injury in the presence of infected soil or dirt in tetanus, infected tranfusion blood or medical instruments in serum hepatitis and sometimes in malaria. In the case of AIDS, while a number of different circumstances will transmit the disease, each requires the introduction of a contaminant into the bloodstream. A disease such as tuberculosis may be transmitted in several ways—by contact (human or animal), through food or eating utensils, and by the air. Control of communicable disease depends upon recognition of the many ways transmission takes place. It must include isolation or even quarantine of persons with certain diseases. Proper antisepsis (see antiseptic) should be observed in illness and in health. Immunologic measures (see immunity) should be utilized fully. Some sexually transmitted infections are associated with cancer (cervical or penile). Education of the population in rules of public health is of great importance both in the matter of personal responsibility (disposal of secretions, preventing contact with the blood of others, proper handling and preparation of food, personal hygiene) and community responsibility (safe water and food supply, sterile blood supply, garbage and waste disposal). Animal and insect carriers must be controlled, and the activities of human carriers must be limited.