Communicable diseases are spread from person to person through various modes of transmission including direct contact, vectors like insects, contaminated food and water, airborne transmission, and indirect contact. Examples discussed include chickenpox, diphtheria, filariasis, AIDS, polio, malaria, measles, tuberculosis, tetanus, rabies, and sexually transmitted diseases. Prevention methods focus on vaccination, hygiene, avoiding contact with infected individuals or carriers, and safe sexual practices.