April 1915 saw the first use of chlorine as a poison gas leading to asphyxiation at Ypres. There were over 15,000 casualties, a third of those fatal. The attack prompted the quick issue of respirators to soldiers. There were two subsequent chlorine gas attacks in 1915, and phosgene was also used. Gas shells were then introduced in 1916, and shells with mustard gas, the most effective of the chemical weapons, first appeared in 1917. Gas poisoning manifested itself by eye and throat irritation, coughing, vomiting, and headache and could lead to bronchitis and pneumonia. Many soldiers recovered from gassing but could suffer permanent lung damage, and, in the case of mustard gas, eye damage. Arthur Hurst records over 160,000 cases of gassed soldiers treated at casualty clearing stations of the British Expeditionary Force.
Major George A. Soper of the U.S. Army, in a 1919 article in The Military Surgeon, reviewed statistical information on the experience of diseases among American troops in 1918 and concluded that:
"The large amount of sickness, which customarily occurs when large numbers of green troops are rapidly mobilized, has not been wholly avoided in this war. The intestinal diseases and malarial infections, which in former wars constituted so conspicuous a cause of disability and death, were practically eliminated. The sickness has been confined almost exclusively to the respiratory type of infections. A large part of the respiratory disease was connected with epidemics, of which the influenza outbreaks which occurred in the spring and early fall were the most conspicuous examples. It is probable that had it not been for the pandemic the death rate would have been little more than one-third of the rate recorded."