One hundred years ago a child born in England could expect to live to 55 if it was a girl, 51 for a boy. A girl born in UK in 2006-8 is expected to live, on average, to 81 and a boy to 77 - this increase is due, in part, to modern medicines.
In the 21st century medicines are available to treat a wide variety of diseases, but most of them only treat the symptoms of the disease - they don't provide a long term cure. Because infectious diseases are caused by pathogens that can be killed, medicines to treat infectious disease aim to not only make you feel better, but actually cure the disease.