You have probably heard of a virus - but you may not be sure what one is. Let's start by making sure you know what a virus is before moving forward. Viruses may appear to be living but are considered non-living because they are incapable of carrying out all life processes. 

Factors that cause viruses to be labeled as non-living:

• not made of cells
• can't reproduce on their own
• do not transform energy
• do not undergo division
• can't synthesize protein

A virus is made of nucleic acid, proteins and sometimes lipids. Visit the Bitesize site, and read the 3 pages on Viruses and Cellular Defence. Take notes on viral structure. 

Viruses enter cells in different ways. For example, bacteriophage (a virus that infects bacteria) enters a cell by piercing the host cell. Eukaryotic viruses enter by endocytosis. Viruses cause two types of infections;

• Lytic - is an infection by a virus that causes a host cell to burst, The Lytic Cycle
• Lysogenic - is an infection that involves the DNA of the virus to combine with the host cell DNA, you can read more on How Viruses Work. 

Take the time to watch the Bozeman Science video, Viruses.