Blood in the urine when you have a cold is usually due to the following three reasons: First, if you have a cold combined with a bacterial infection, the bacteria will invade the respiratory tract, the digestive tract and the urinary tract when the body’s immunity is low. When the invasion of the urinary tract, it will retrograde cause urinary tract infection, and lead to acute pyelonephritis and acute cystitis. At this time, there will be frequent urination, urinary urgency, painful urination with hematuria. Secondly, if the blood is concentrated when you have a cold and drink too little water, the urine will be too concentrated and the symptoms of blood in urine will also appear. Third, untimely hydration or fire and anxiety can trigger the discharge of existing kidney stones down the ureter. When sediment-like stones or small foci of kidney stones are discharged, it will cause transient hematuria of the naked eye.