**2. Antiviral drugs**

Although viral infections have been disclosed since earliest times, it was only during the nineteenth century that scientists were able to isolate "the filterable particles", later called viruses. Viruses are the infectious agent that reproduces only inside the living cell and they can infect all types of living cells and direct the cell to produce more viruses. They are smaller than the bacteria's. The entire infectious virus particle, called a virion, consists of the nucleic acid and an outer shell of protein. Antiviral drugs are classified as the medicines which are mainly used to treat viral infection. Many of the antiviral drugs target the specific viruses and they are considered as the broad spectrum drugs. Most viral infections rectify spontaneously in immunocompetent individuals. The objective of antiviral therapy is to diminish the symptoms and infectivity additionally to shorten the duration of illness [7]. These drugs react by arresting the viral replication cycle at numerous stages. Most of the antiviral drugs currently available are used to treat infections caused by HIV, herpes viruses, hepatitis Band C viruses, and influenza A and B viruses, and C viruses, and influenza A and B viruses etc. Designing effective and safe antiviral drugs is difficult as viruses use host cells to replicate. It is back-breaking to find targets for drugs that would affect only viruses leaving host cells. Viral variation is also one of the reasons posing difficulty in the development aof antiviral drugs and vaccines. Furthermore, antiviral drugs are classified as non-retroviral and retroviral drugs which act on the different types of viruses and stop their further growth or their replication and these results in the cure of viral disease [8]. Coronavirus 2019 (COVID 19) is highly infectious disease triggered by SARS-CoV-2 (severe acute respiratory syndrome coronavirus 2 causing nearly 2.9 million deaths worldwide Currently, a variety of antiviral drugs are in clinical trails to treat SARS-CoV-2 which inhibit viral replication.
