WHAT IS A VIRUS?????
A virus is referred as an infectious agent that can only replicate inside the living cells of an organism i.e. a virus is something which can not at all grow or replicate by its own. It always needs a living cell for its replication process. It is a microorganism which cannot be seen by naked eyes and can infect any life form. It can be infectious for humans, plants and even for other microorganisms like bacteria and archea. Viruses infecting bacteria are known as bacteriophage. Viruses are not restricted to a place and they can be found everywhere at every place of ecosystem whether land, or water or in air. They can cause various infections including air-borne, water-borne or even food-borne. The science dealing with the study of viruses is known as Virology and it is a branch of microbiology. A complete virus particle ranges in size from about 10-400nm in its diameter.
Viruses are near to dead when outside the living cell but once entered any living cell of an organism, they are forced to replicate using the life machinery of that particular organism and thus they produce thousands of their multiple copies and in this way infect the organism. Outside the living cells they are present in the free, independent form which may also be known as a virion.
There are 3 main parts in the structure of a virus i.e. –

1. Genetic core which is also known as nucleic acid core containing all the genetic material whether DNA or RNA, but not both. It is known as genome.
2. A protein coat, which is also known as capsid which surrounds the genome of a virus particle.
3. An envelope which is made of lipid. It is an external coat surrounding the genome as well as capsid.

VIRUS TRANSMISSION
Transmission of virus particles is important for them to survive because as discussed above they can only replicate themselves inside a host living organism. The virus transmits from one organism to another in order to survive, reproduce and continue their species. The effectiveness of the transmission of viral particle depends on 2 main factors i.e. the concentration of virus and its route of transmission. More concentration of virus leads to more transmission.
There are several ways by which a virus particle may get transmitted from one organism to another.

1. Blood – Virus particles can get transmitted through the blood. The one way is direct viral infection in blood and the other way is by arthropods like dengue or malaria is transmitted. Arthropods bite one organism and collect viral particles from them and then when they bite other organism, the same viral particles are being transmitted to the next organism and this way transmission and infection occurs. Another way is direct viral infection in blood which can be via direct infected blood exposure to a healthy individual. It may be transmitted via sexual contacts with infected person like HIV is transmitted.
2. Saliva – It is the most commonly seen in kissing the infected individual. The saliva contains the viral particles and thus they are transmitted to healthy individual.
3. Respiratory secretions – If any infected individual sneezes, or coughs or in any other way its respiratory secretions come in contact with the healthy individual, he may get infected by the same. It may also occur by singing or even breathing.
4. Feces – This is not a very common method in developed countries but can infect those who do not take sanitary actions after using toilets. The virus particles secreted in feces can infect other healthy individuals if they come in contact with them.