Many organisms live in and on our bodies. They're normally harmless or even helpful, but under certain conditions, some organisms may cause disease. Some infectious diseases can be passed from person to person” (Mayoclinic 2009). Viruses are microscopic infectious agents that can only be seen with an electron microscope. There categorised as non-living because they don’t self – reproduce, grow, feed or produce waste. They don’t move around unless they latch onto organisms, wind or water. Viruses are not cells. There made of DNA and coated in protein. Viruses attach themselves to a suitable host sell then program the original cell to reproduce more viral particles, when this becomes too much for the cell to handle it bursts open releasing all the viral particles into the organism. Some viruses invade the cell but then choose to remain inactive for periods of time, for example the herpes simplex virus you may carry the genes for the virus but you don’t get them constantly they come and go. As well some viruses aren’t programmed to kill but to become cancerous. There not stopped by antibiotic but rely on the body’s immune system to kill off the