They are minute structures and can be studied with the help of electron microscope.
Size :The normal size of virus is 10 to 200 nM. The size may be upto 300 pM. The plant viruses are smaller than the animal viruses.
Shape :They have different shapes such as roughly spherical (pleiomorphic)
eg. – Influenza virus, mumps virus, Icosahedral eg – Herpes virus, Helical eg.-TMV etc.
Chemical Composition :The fundamental structure of all the viruses is similar. Each virus consists of central core of DNA or RNA which is bounded by a protein covering. The base pairs (nucleotides pairs) varies from 1000 to 2,50,000. But it is constant in a specific virus.