The United Nations has warned of an “alarming” decrease in the number of children receiving life-saving vaccines around the world. The drop included vaccinations for preventable diseases such as diphtheria, tetanus and measles, the U.N. said in a statement. The U.N.’s World Health Organization and UNICEF blame the decrease on the change in routine health care caused by the coronavirus crisis. A vaccine is a biological preparation that provides active immunity to an infectious disease. A vaccine usually contains a substance that is similar to the disease-causing illness. It tells the body's immune system to recognize the substance as a threat and destroy it.