What can cause someone cough up blood? Is that serious?

Frank blood is. serious. Per Mayo Clinic, possible reasons for coughing blood include: chronic bronchitis, bronchiectasis, Pulmonary Tuberculosis, Chronic obstructive pulmonary disease, lung cancer, cystic Fibrosis, pneumonia, abscess of lung, emphysema,pulmonary embolism, chest trauma, parasitic infections. See: http://www.mayoclinic.org/symptoms/coughing-up-blood/basics/causes/sym-20050934.