Viruses need hosts to survive, and for this reason they are called obligate parasites. They take over the machinery of whichever cell they infect and use its resources to survive (reproduce, etc.). Certain viruses infect certain types of cells, which is why when we have a viral infection only some parts of our body are infected. Cold viruses can infect throat cells, HIV affects T-cells of the immune system, and the latest Ebola virus kills basically any cell in its path. Some pathways of viruses are unknown, but usually all of them undergo the lytic cycle. First, the virus attaches to the host cell and then releases its genetic material (double stranded DNA/RNA or single stranded DNA/RNA) into the cell. Next, the injected genetic material recruits host cell enzymes which aid in making more parts of the virus. The virus assemble into more particles and then the cell breaks to release them. These cycle keeps happening. Some viruses can stay in the lysogenic, dormant cycle, where the genetic material is inserted but they are not ready to break out with new particles.