Ominous

It watched and waited for several years.  A human stalker, the cell could have died but it was never attacked.
The DNA of a single cell had somehow became scrambled. Its original code was lost and was replaced with a new genetic blueprint. It would no longer be a normal human cell. 
This newly born cell split and divided like all cells are programed to do.  It was an evil radical and frightenly aggressive.
Sometimes the cells would go into a resting phase. But mostly they grew.  The four became eight and the eight became sixteen. . It took several years and the aggressors numbered in the millions. The microscopic blob  soon found a food supply and imbedded itself deeply into the blood vessels and nerves of the shoulder. Like a human embryo, it implanted eagerly and quickly. Soon it had  a placenta manufactured. It stole nutrients from the hosts body and became a silent parasite. Whenever the middle aged woman took vitamin pills, the abnormal cells helped themselves. They became healthy and strong and multiplied faster and faster.