In this video, we're going to look at what viruses are. Viruses are type of microorganism. They are too small to be seen by the naked eye, much smaller than bacteria and about a hundred times smaller than human cells. They're coming in many different shapes and are present wherever they are so soothing. In fact, viruses are the most common biological unit on earth, outnumbering all other types of the virus combined. Viruses can infect humans, all animals, plants and even bacteria. Viruses are very simple. They are made up of a protein code surrounding a strand of genetic material. The genetic material can either be DNA or RNA. Sometimes a membrane called an envelope surrounds a virus particle. This envelope isn't made by the virus, but is actually stored in from the membrane of the host cell. This is a great strategy by the virus. It makes it harder for the host cell to identify the virus as foreign. As we just saw, viruses aren't made up of cells. In fact, they aren't really living. They are halfway between a living organism and a chemical. Because they aren't living, we say they are different types of virus rather than saying different species of virus, some common types of viruses or influenza and HIV or AIDS. So if viruses aren't living, how do they cause illness? Viruses are parasites that can only reproduce in other living cells. They're in the host cell and hijack the host's genetic machinery. They make copies of their own viral genetic material instead and produce lots more virus particles. There's a lot of virus particles that we made. The host cell dies and the viral particles are released to infect more cells. The released viral particles can also spread to other people. There are actually two different ways in which virus attacks the host cell, which we'll look at in more detail in this video. Viruses cause many human diseases, including skulls, influenza, rabies, yellow fever, HIV or AIDS, pneumonia, bird flu, Zika, Ebola and cancer causing HPV. There are just a few examples. Fortunately, the human immune system is very good at dealing with viral invaders. Sometimes our immune system will recognize a virus as an intruder and destroy the virus before it gains entry into a cell. Even often infection begins, our immune system destroys the virus and the person recovers. Sometimes, the immune system cannot destroy the virus's plasma and the virus can cause permanent damage or death. In 1918, the Spanish flu is thought to have caused up to 50 million deaths worldwide. Scientists have developed vaccinations against lots of viruses, which have even led to the eradication of some viruses like smallpox as human hosts become immune. However, some viruses like HIV have proved impossible to develop any kind of vaccine for. Did you know that antibiotics do not work for viruses? They are only for bacterial infections. If you have a cold of flu, you shouldn't be taking antibiotics. They won't make any difference. There are some antiviral drugs that have been developed, such as the one that dramatically prolongs the life of people affected by HIV. So there we are viruses. They are incredibly simple organisms, but are extremely effective at causing infection.