The Dutch term “Q fever” is derived from the English “Query fever,” which can be loosely translated as “fever with a question mark.” The term dates back to 1937, when a mysterious feverish illness broke out among slaughterhouse workers in the Australian city of Brisbane. Researchers initially did not know what was causing it. After intensive research, microbiologists Macfarlane Burnet and Herald Cox discovered the bacterium that caused the disease. That bacterium has since been named Coxiella burnetii. How do you become infected? Q fever is what’s known as a zoonosis: a disease that spreads from animals to humans. In the Netherlands, goats and sheep are the main sources of infection for humans. The bacterium spreads through the air. People become infected by inhaling it in the vicinity of infected animals. Infected animals shed the bacterium through the placenta and amniotic fluid, as well as through urine, feces, and milk. The bacterium can survive for months to years in dried-out material and is resistant to heat and many disinfectants. Most infections occur between February and May, when goats and sheep give birth. Q fever is extremely contagious; even a small amount of bacteria is enough to cause illness. What are the symptoms? After infection, it can take two to three weeks for symptoms to appear. This happens in about 40% of those infected; the rest show no symptoms. At first, the symptoms resemble those of severe flu: high fever, headache, joint pain, and severe fatigue. Many people recover afterward. Some people develop complications, including pneumonia, liver inflammation (hepatitis), and meningitis. Older adults, pregnant women, and people with a weakened immune system or a heart valve defect are at higher risk of a severe or fatal course of the disease. The mortality rate from acute Q fever ranges from 1 to 2%. In approximately 2% of patients, the bacterium remains in the body. This is known as chronic Q fever. In such cases, the bacterium can cause life-threatening infections of heart valves, vascular prostheses, and joint prostheses. The mortality rate for chronic Q fever is high: according to the Dutch database maintained since the outbreak between 2007 and 2010, 29% of patients with chronic Q fever die. Q-Fever Fatigue Syndrome A distinct and common complication is Q-fever fatigue syndrome, abbreviated as QFS. Approximately one in five people who have had an acute Q-fever infection develop this syndrome. In QFS, no live bacteria can be detected, but the immune system remains overactive. The symptoms are severe: extreme fatigue, muscle pain, joint pain, concentration and memory problems, sleep disturbances, headaches, blurred vision, night sweats, shortness of breath, irritability, depressive symptoms, and reduced exercise tolerance. Daily life, work, and social interactions come under severe strain. QVS shares many similarities with other conditions and infections, such as long COVID, Lyme disease, and mononucleosis. It is not yet known whether a complete recovery is possible. The Dutch Outbreak The largest Q fever outbreak ever occurred in the Netherlands between 2007 and 2010. Officially, more than 4,000 cases of infection were confirmed through laboratory testing. Researchers at the RIVM estimate that the actual number of infections was between 40,000 and 50,000. Many of these people continue to experience symptoms to this day