1. All the viruses are ultramicroscopic.
2. All viruses are obligatory intracellular parasites.
3. The viruses can’t be grown in artificial media in any case and the living cell seems essential for the multiplication of viruses.
4. They are crystalline nucleo-proteins of very high molecular weight.
5. They can live only in a living cell.
6. They can infect healthy plants just like bacteria and fungi.
7. They are too small to be observed under visible light.
8. They retain infectivity even in very low concentartions.
9. They can be crystalized like a chemical substance.
10. They can be sedimented like proteins.