Modern laboratory medicine knows everything. It knows blood and hormone levels, can identify germs and fungi, and can check all organs. But sometimes, the use of a fine nose is enough to recognize the first symptoms of disease. Sour or sweet? Rotten or fresh? Musk, apples, or ammonia? With certain diseases, the body produces metabolic products that are excreted through sweat, breath, or urine, and have a strong inherent smell. Some of these smells could be helpful as early warning signals and might even make complicated tests unnecessary in the future.