22 Hard-To-Detect Illnesses, From The Commonplace To The Truly Bizarre

10. Deep Vein Thrombosis

Blood clots in the lower leg often go unnoticed, but if they go untreated they can spread to your lungs and cause damage. Most people who suffer from DVT do not experience any symptoms, which makes the disease especially dangerous, as it can lead to a life-threatening pulmonary embolism.