Why are viruses important?
- Viruses cause disease (hemorrhagic fever, immunodeficiency, cancer, dementia, organ failure, etc.)
- Viruses can be used to treat disease (gene therapy, tumor-killing oncolytic viruses, bacterophage therapy for bacterial infection, etc.)
- Viruses can be used to control of pests (insects, rabbits, etc.)
- Viruses can be used for biotechnology applications (protein expression, phage-display libraries, induced pluripotent stem cells, etc.)
Viruses are the most abundant biological entity in the world and are dependent upon their hosts for resources and often take advantage of the host's biological machinery to replicate (viruses do not reproduce).
Viruses infect every known type of life - the three domains of life: Bacteria (there are more cells of these on/in your body than your own cells), Eukaryota (that's us), and Archaea (A lot like bacteria, but are actually closer relatives to us) are all infected by viruses.
There are even small viruses (Sputnik virophage - about 50nm) that "infect" very large viruses (Mamavirus - about 400nm) at the same time the large viruses are infecting a larger cell (an amoebae).
Although viruses are not classified as life, they are still subject to the principles of natural selection and have co-evolved with their hosts. That means that viruses are selected for being the best at spreading through their host population (this does not mean that they will be the best at killing their host).
There is some disagreement on how to define viruses, but the formal definition I would give is: A biological entity that is dependent upon its host to replicate and requires the assembly/disassembly of capsid proteins to produce replication-competent derivatives of itself.
Summary: Viruses are small and usually much simpler than cellular life. All viruses need a host to infect in order to replicate and pass its genes to future generations. Due to natural selection, viruses become optimized for being able to spread through its host population.