Infectious disease occurs when the body is penetrated by microscopic organisms, which then multiply, potentially causing a wide range of physical symptoms. In recent decades, tremendous progress has been made in controlling, or in some cases completely eradicating various infectious diseases, but even so, humankind remains at risk to a wide array of infections. In some cases, new bacteria evolves, creating potentially dangerous infectious diseases, and in other cases, previously controlled bacteria may become resistant to treatment. Infectious diseases are commonly spread through skin contact, inhalation, or ingestion of contaminated food or water.
Immunization programs have helped control the spread of many infectious diseases, including, diphtheria, influenza, measles, mumps, pertussis (whooping cough), polio, tetanus, and varicella (chicken pox). Those traveling abroad may require additional vaccinations for such diseases as cholera, yellow fever, or typhoid fever.
Antibiotics remain an effective weapon in the war against infectious diseases, but physicians must use them carefully, as overuse may cause bacteria to become drug-resistant, making these treatments ineffective for future generations.