Peste vs. piaga
The Italian words 'peste' and 'piaga' both relate to the concept of plague, but they have distinct uses and connotations in the language.
Peste
'Peste' specifically refers to the infectious disease plague, caused by the bacterium Yersinia pestis. It can also be used figuratively to describe something extremely annoying or troublesome.
La peste nera decimò la popolazione europea nel XIV secolo.
(The Black Death decimated the European population in the 14th century.)
Quei bambini sono una peste, non stanno mai fermi!
(Those children are a plague, they never stay still!)
Piaga
'Piaga' has a broader meaning, referring to a wound, sore, or more generally, a scourge or widespread problem affecting society.
La corruzione è una piaga che affligge molti paesi.
(Corruption is a plague that afflicts many countries.)
Il medico ha esaminato la piaga sulla gamba del paziente.
(The doctor examined the wound on the patient's leg.)
Summary
While 'peste' specifically refers to the disease plague or metaphorically to a nuisance, 'piaga' has a wider application, encompassing physical wounds and societal issues. 'Peste' is more precise in its medical context, while 'piaga' is more versatile in its usage across different contexts.