What description best explains why viruses can become more pathogenic to host that were not affected by the virus before?

New cells are naive to the infectious cells who attack it or they are not well prepared to deal with the different scenarios. But, the cells who are attacked before has the set or sequence of the viral or bacterial genome strand been identified by them, which leads to more safety or protection from these foreign bodies.

Explanation:

The viruses and other infectious material enters and attacks the host cell, by breaching its membrane wall and installing or leaving a gene of its own inside the cell. Which then combines with the genome of the cell and it goes through the process of replication, translation etc,along with the host cell machinery. Which then spreads the specific gene strand more in the environment

After the genome enters the host cell at first it does not recognizes the strands or foreign cells, as they cover there body with a camouflage sort of membrane and they look more like the body cells.

The organisms detects the genome of the infections cells or strand, as they store the data about it in its server or database. As if the next time they were under attack then precautions will be there by the host cell to deal with it.

As for the cell who are never attacked before will be less safe to deal with these foreign bodies.