The flu (influenza) virus is spread by coughing or sneezing. When a sick person coughs or sneezes they expel thousands of virus particles. These can get into the body of another person via the nose (as they inhale) or through contact with mucous membranes (e.g. if they touch infected surfaces and then touch their eyes). The virus moves through the airways/ mucous membranes to the respiratory tract. It binds to the surface of cells in the respiratory tract and releases it’s genetic material RNA into the cell. It actually hijack’s the cell and uses the cell’s mechanisms to reproduce. The virus genetic material in the cell makes many many copies of itself. Eventually the many virus copies burst out of the hijacked host cell, killing the cell and releasing many viruses to colonise new cells. The process continues until there are so many viruses in the body. This triggers the immunity to work very hard to kill the virus and causing a lot of inflammation. This immune response gives a person fevers and makes them feel very sick.