Virus, a minute particle having DNA or RNA enveloped in a protein coating. These small biological agents are even 100 times smaller than the smallest bacterium. There are thousands of viruses in the air around you but they infect only when they invade a cell. They can infect a plant, animal or human being. When they lodged a cell, they take control of its replicating machinery and starts replicating itself making its copies. Those copies may invade other cells and cause infection. When the copying remains unchecked it may lead to the death of the organism, whose cell was invaded.
Luckily, the human body is made so that it can fight against the invasion of viruses. T-cells of the immunity system works to fail the action of the virus. The antibodies produced by the immune system get glued with the invaded viruses and render their action failed. A cell starts RNA interference when the reduced the effect of viral RNA and hinders its working. But the viral RNA works more efficiently than a human immunity system. In some cases, viral RNA or DNA surpasses the immunity system and starts infecting the invaded cell.
A virus is able to cause a long list of diseases in plants. Viruses can cause barley yellow dwarf, watermelon mosaic, potato mop-top, peach yellow bud mosaic, tomato spotted wilt and other diseases in plants. Viruses are able to cause African swine flu, African horse sickness, avian influenza, foot and mouth disease, Newcastle disease and many other diseases in animals. Similarly, viruses are responsible for many diseases in human beings including different kinds of flu, ebola, smallpox, measles and mumps, hepatitis, COVID-19, and many others.
Some viruses are zoonotic i.e., they can transfer from animals to humans. According to WHO, 60% of the raising diseases are zoonotic. The well-known virus of these days named new coronavirus is believed to be transferred from an animal (bat or civet cat) to a human. Coronaviruses are the group of viruses responsible for MERS-CoV (Middle East Respiratory Syndrome) and SARS (Severe Acute Respiratory Syndrome) and the new strain of the virus is found in 2019 which is responsible for COVID-19. The COVID-19 is pandemic and has caused over 6000 deaths worldwide.