Which effect of the plague does the narrator in The Decameron describe as even worse and almost incredible as he tries to convey the horror of that time period?

The plague itself was terrible. The narrator described the swellings, the size of an apple or an egg, that would appear on people's groins or underarms and then spread all over the body. The body of the infected would show dark and livid spots, and three days later the person would die.
Adding to the horror was the extremely infectious nature of the disease and the fact that it could be transmitted from human to animal and animal to human, as well as through the clothes, food, or touch of a contaminated person.
This led to what the narrator characterized as the worst effect of all: people became hardhearted and solely concerned with saving themselves. People cut themselves off from contact with other people, even their close relatives, leaving them to die alone.