The big difference between viruses and all else, is that fact that viruses are so small they can not be viewed without the help of an electron microscope. This is because viruses are, on average, smaller than a regular wavelength of visible light. In effect, the viruses can hide between light waves, thus making them colorless. They can not be seen by the naked eye or a regular microscope. Viruses are so small in fact, that the largest virus is equal in size to the smallest bacteria. The smallest virus measures only 20 nanometers in length. Because of their incredibly small size, viruses are extremely hard to study and understand.
Shape is also a defining characteristic of viruses. The basic shapes viruses tend to take are rods, filaments, crystals, helixes, polyhedrons and spheres, with added extensions. Almost all human viruses are close to being spherical. Every virus carry proteins and nucleic acids in a protective coat. This protective membrane is called the capsid. Extensions on any virus are called antigens. The antigens allow viruses to identify, attack, and enter its target host. Viruses are not classifiably alive or dead. They seem to be in limbo between each state. Viruses exist this way because they are strictly parasites. That is, they can not survive and thrive without a host or group of host cells. The hosts provide viruses with all the chemicals and molecules they need to survive and reproduce. You might think of viruses' as robots that need to take over a factory to make more of themselves. Without that, the viruses are dormant. Viruses can lie dormant within any host or environment until the proper conditions for their activity are provided. This is why we sometimes say that viruses have incubation periods of certain lengths. Some viruses are also classified as 'persistent viruses'. Such viruses can enter and exit host cells without killing them. Even so, each different virus is stimulated by different conditions and they all have different, specific functions they affect in their host. They are mysterious and dangerous creatures.