To stay healthy, your cells must continuously perform a wide variety of chemical tasks. Your DNA genes, in the nucleus of every cell, help orchestrate this activity.
Cells Work Hard
A cell uses its genes as a kind of biochemical cookbook: whenever it has to do anything, a cell looks up the recipe in its DNA. It then uses this recipe to cook up whatever proteins it needs to get the job done.
Your cells are hard at work right now, creating thousands of tailor made proteins for thousands of different biochemical jobs. Sometimes, however, things can go wrong.
When a virus attacks, it injects its own DNA into one of your cells. Like your own genes, the viral DNA contains recipes. Unfortunately, these recipes are only good for making new viruses.
No matter what your cell was doing before, it will stop its healthy functioning and make only viruses, following the viral recipe until it runs out of raw materials. Then your once healthy cell bursts open, releasing new viruses to infect more cells.
How Cancer Works
Cancer is another problem that can develop through a cell's recipe reading process. Each cell contain dozens of genes that regulate its healthy growth and reproduction. If these genes become damaged or altered, the results can be dangerous.
Instead of growing in a healthy fashion, the cell grows and divides at an alarming rate. A cancerous tumor is made up of cells like these, cells whose growth and reproduction genes are altered.
So viral infections are caused when alien recipes are added to a cell's genetic cookbook, and cancer is caused by errors within your own genetic recipes.