When a cell becomes infected, the DNA inside the cell is altered and this causes it to lose its function.
Cells with altered DNA become vulnerable to pathogens and reach a point where they synthesize the DNA that produces pathogens. Then, these infected cells turn into factories that continuously print germs.
This is why NK cells are so important. NK cells circulate throughout the body, finding only infected cells and inducing their death. It attaches to abnormal cells and injects water and salt to disturb the balance of the cells, causing cell necrosis.