Viruses and viral diseases
The virus is an infectious agent that does not have a cellular structure, and is capable of reproducing only within cells of living organisms. For this they are referred to obligatory parasites.
They exist wherever there is life. And they are at the same time the most numerous form of life. Their study is practiced by science of virology.
Photo by Razza Mathadsa, link to the original (photo has been changed).
The structure of viruses
The main parts of all viruses are genetic material (genome) represented as DNA or RNA, and capsid (Protective protein shell of the genome). Sometimes they may have one or more additional lipid shells that perform a protective function. But precisely the presence of the genome and the capsid is a hallmark of these parasites. Yes, there are infectious agents, similar to viruses, but without capsids. They are called viroids. They are also a very common form of life. However, unlike viruses that affect all living organisms, they affect plants, for the most part.
But infectious agents that do not contain DNA or RNA are called prions. And for humans, they are deadly, as they affect the nerve tissue (including those contained in the brain). There are no medications from such diseases.
Infectious agents are very small. So that they can not be well considered under a light microscope. So, with the naked eye we do not see even bacteria, and viruses are less than their hundreds of times, do you imagine?
Hypotheses of origin
No one can reliably say where the virus came from. It is believed that they exist since the appearance of living cells, but this is only an assumption.
Many hypotheses of their origin have been put forward, but the main, most probable, are only three of them:
According to a regressive hypothesis, these parasites used to be very small cells that parasitized cells in living organisms. But, unnecessarily, a number of genes have disappeared, due to which substantially changed. It remains unclear why these degraded cells are not alike to living cells.
The cell-based hypothesis suggests that the viruses arose as a result of the release of a DNA or RNA fragment from the cell’s genome and subsequent changes. But there was no explanation for how parasites took their current look. And in particular, whence came the same capsid (protein shell), as well as lipid shells.
But the supporters of the co-evolution hypothesis believe that the infectious agents were formed together with living cells, and had approximately the same structure as now. But, in this case, they are seen as independent non-cellular forms of life, and this is not so. Parasites can not exist without cells of living organisms.