What are some examples of viruses?

Viruses are small infectious agents that hijack host cell's replication machinery to replicate inside a living host cell. Unlike bacteria (another infectious agent), it is debatable whether viruses are considered a form of life or not due to the fact that they miss key 'properties of life' - those typically observed for typical living cells. For instance, viruses only replicate within the host - by using the host's DNA replication machinery; and uses the hosts' metabolic processes, which they also lack. This is still, however, debatable. Viruses have their own genetic material and are able to undergo natural selection.

There are different types of viruses, and classifications also vary. They can be classified according to morphological form or type of genetic material transmitted. Various morphologies include helical, icosahedral, prolate, envelope, and complex. The chickenpox virus has a lipid envelope. The adenovirus that infects humans is icosahedral. The Zika virus is both icosahedral and enveloped.

The genetic material also varies - they may carry RNA or DNA, and these can be double-stranded or single-stranded. Single-stranded types can be the sense or anti-sense strand.

• dsDNA - examples include adenovirus, herpesvirus
  


• ssDNA - include parvovirus
  


• dsRNA - reoviruses, nonpathogenic for humans but have been used as model for viral study


• (+)ssRNA - sense-strand, rhinovirus, causes colds.


• (-)ssRNA - anti-sense, rhabdovirus, rabies.


• ssRNA-RT -sense-strand, retrovirus (they have DNA intermediates reverse-translated from RNA) - example is Rous sarcoma virus that causes sarcoma.


• dsDNA-RT - hepadnavirus
  

---short answer

There are a lot of viruses that infect hosts ranging from bacteria (bacteriophages) to humans. Some are nonpathogenic to humans but are relevant for research (protein expression in insect cells, for instance, or in studying the general life cycle of viruses). Some, however, are highly known for infecting humans. Dengue virus is an example, or more currently the Zika virus. HIV has also plagued the human race causing AIDS.