infectious & contagious

infectious: an infectious illness can be passed from one person to another, especially through the air you breathe (LDOCE)

contagious: a disease that is contagious can be passed from person to person by touch (LDOCE)

a contagious disease

an infectious disease

上述兩個都有被翻作「傳染性疾病」，但看看LDOCE的解釋，contagious算是接觸性傳染的，而infectious是特別指由呼吸，但應該包含其他非接觸之途徑（像水、食物等等）傳染的。