Once inside the cell, the viral enzymes take over those enzymes of the host cell and begin making copies of the viral genetic instructions and new viral proteins using the virus's genetic instructions and the cell's enzyme machinery (see How Cells Work for details on the machinery). The new copies of the viral genetic instructions are packaged inside the new protein coats to make new viruses.
Once the new viruses are made, they leave the host cell in one of two ways:
- They break the host cell open (lysis) and destroy the host cell.
- They pinch out from the cell membrane and break away (budding) with a piece of the cell membrane surrounding them. This is how enveloped viruses leave the cell. In this way, the host cell is not destroyed.
Once free from the host cell, the new viruses can attack other cells. Because one virus can reproduce thousands of new viruses, viral infections can spread quickly throughout the body.
The sequence of events that occurs when you come down with the flu or a cold is a good demonstration of how a virus works:
- An infected person sneezes near you.
- You inhale the virus particle, and it attaches to cells lining the sinuses in your nose.
- The virus attacks the cells lining the sinuses and rapidly reproduces new viruses.
- The host cells break, and new viruses spread into your bloodstream and also into your lungs. Because you have lost cells lining your sinuses, fluid can flow into your nasal passages and give you a runny nose.
- Viruses in the fluid that drips down your throat attack the cells lining your throat and give you a sore throat.
- Viruses in your bloodstream can attack muscle cells and cause you to have muscle aches.
Your immune system responds to the infection, and in the process of fighting, it produces chemicals called pyrogens that cause your body temperature to increase. This fever actually helps you to fight the infection by slowing down the rate of viral reproduction, because most of your body's chemical reactions have an optimal temperature of 98.6 degrees Fahrenheit (37 degrees Celsius). If your temperature rises slightly above this, the reactions slow down. This immune response continues until the viruses are eliminated from your body. However, if you sneeze, you can spread thousands of new viruses into the environment to await another host.