The study suggests the coronavirus mutated in Italy, which led to an increased viral load and increased transmissibility, but did not increase the virulence.

Italy has a high number of elderly people, who are more like to have chronic underlying infections; and a high rate of rare diseases, which are likely caused by common intracellular pathogens, such as psittacosis, trachoma, or strep. Italians are at higher risk of chronic psittacosis from contact with infected birds; and are also impacted by environmental contamination.

The coronavirus may have mutated in the Italian population, by combining with underlying chronic intracellular pathogens, common in the Italian population.

https://www.washingtonpost.com/science/2020/06/29/coronavirus-mutation-science/