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Human and animal diseases are often caused by viruses or bacteria. Over the past two hundred or so years, vaccines have eradicated some of these diseases. Others have returned to haunt humans with new and ever-mutating strains, or revived when vaccination programs were interrupted. Communicable diseases may spread in different ways: through blood, air, feces/urine, food, or water. The World Health Organization (WHO) and the Center for Disease Control (CDC) keep constant watch over the most potentially dangerous diseases and the most likely threats to various world populations.