Usually the first action a virus or a worm performs is:

Answer;

-Replicate itself

Explanation;

Usually the first action that a virus or worm performs is to replicate itself. This attempts to ensure that the virus or worm spreads before being detected by a payload.

Viruses or worms are very simple microbes, they can't do much by themselves. Viruses need a host, another living organism that gives them everything they need to work.

-Viruses take any chance they can to find a host.  They get inside the host's cells and take it over.  They use the host cells machinery to make lots of copies, so many that the cell bursts and infects other cells around it.

any abiotic or biotic factor in an ecosystem that causes a population's size to slow or decrease is a limiting factor.