Since its outbreak in China at the end of 2019, the new coronavirus disease (COVID-19)
was characterized by both easy spreading and high mortality. The latter proved to be way more elevated
in the North of Italy -with a peak of 18.4% in region Lombardia and even 31% in the city of
Bergamo and surrounding county- than in the rest of the world. In an attempt to conceptualize the
reasons for such a dramatic situation, four key elements have been identified: COVID-19 itself, old
age, lung disease, and heart failure. Their harmful combination has been named “The deadly quartet”.
The underlying risk factors, among which a lot of them are distinctive features of the population
in northern Italy, have been summarized as “unmodifiable”, “partially modifiable”, and “modifiable”,
for the sake of clarity. Up-to-date scientific evidence in this field has been described in the
form of a narrative and easy-to-read review.