Molecular virology
Molecular virology is the study of viruses on a molecular level. Viruses are submicroscopic parasites that replicate inside host cells. They are able to successfully infect and parasitize all kinds of life forms- from microorganisms to plants and animals- and as a result viruses have more biological diversity than the rest of the bacterial, plant, and animal kingdoms combined. Studying this diversity is the key to a better understanding of how viruses interact with their hosts, replicate inside them, and cause diseases. Viral replication. Viruses rely on their host to replicate and multiply. This is because viruses are  unable to go through cell division, as they are acellular–meaning they lack the genetic information that encode the necessary tools for protein synthesis or generation of metabolic energy; hence they rely on their host to replicate and multiply. Using the host cell's machinery the virus generates copies of its genome and produces new viruses for the survival of its kind and the infection of new hosts. The viral replication process varies depending on the virus's genome. Classification. In 1971 David Baltimore, a Nobel Prize-winning biologist, created a system called Baltimore Classification System.According to this system , viruses are classified into seven classes based on their replication strategy: Replication cycle. Regardless of the differences among viral species, they all share six basic replication stages: Attachment is the cycle's starting point and consists of specific bindings between anti-receptors (or virus-attachment proteins) and cellular receptors molecules such as (glyco)proteins. The host range of a virus is determined by the specificity of the binding. The attachment causes the viral protein to change its configuration and thus fuse with the host's cell membrane; thereby enabling the virus to enter the cell. Penetration of virus happens either through  membrane fusion or receptor-mediated endocytosis and leads to viral entry. Due to their rigid cellulose-made (chitin in case of fungal cells) cell walls, plants and fungal cells get infected differently than animal cells. Often, a cell wall trauma is required for the virus to enter the cell. Uncoating is the removal of viral capsid, which makes the viral nucleic acids available for transcription. The capsid could have been degraded by either host or viral enzymes, releasing the viral genome into the cell. Replication is the multiplication of virus's genetic material. The process includes the transcription of mRNA , synthesis, and assembly of viral proteins and is regulated by protein expression. Assembly process involves putting together and modifying newly made viral nucleic acids and structural proteins to form the virus's nucleocapsid. Release of viruses could be done by two different mechanisms depending on the type of virus. Lytic viruses burst the cell's membrane or wall through a process called lysis in order to release themselves. On the other hand, enveloped viruses become released by a process called budding in which a virus obtain its lipid membrane as it buds out of the cell through membrane or intracellular  vesicle. Both lysis and budding processes are highly damaging to the cell, with the exception of retroviruses, and often result in the cell's death. Viral pathogenesis. Pathogenicity is the ability of one organism to cause disease in another. There is a specialized field of study in virology called viral pathogenesis in which it studies how viruses infect their hosts at the molecular and cellular level. In order for the viral disease to develop several steps need to be taken. First, the virus has to enter the body and implant itself into a tissue ( e.g. respiratory tissue). Second, the virus  has to reproduce extracellular after invading in order to make ample copies of itself. Third, the synthesized viruses must spread throughout the body via circulatory systems or nerve cells. With regards to viral diseases it is essential to look at two aspects: (a) the direct effect of virus replication; (b) the body's responses to infection. The course and severity of all viral infections are determined by the dynamic between the virus and the host. Common symptoms of virus infections are fever, body aches, inflammation, and skin rashes. Most of these symptoms are caused by our immune system's response  to infection and are not the direct effect of viral replication. Cytopathic effects of virus. Often it is possible to recognize virus-infected cells through a number of common phenotypic changes referred to as cytopathis effects. These changes include: Viral infections. There are five different types of viral infections: Prevention and treatment. Viral vaccines contain inactivated viruses which have lost their ability to replicate. These vaccines can prevent or lower the intensity of viral illness. Developing vaccines against smallpox, polio, and hepatitis B over the past 50 years has had a significant impact on world health and thus on global  population. Nevertheless, there have been ongoing viral outbreaks ( such as the Ebola and Zika viruses) in the past few years affecting millions of people all around the globe. Therefore, more advanced understanding of molecular virology and viruses are needed for the development of new vaccines and the control of ongoing/future viral outbreaks. An alternative way to treat viral infections would be antiviral drugs in which the drug blocks the virus's replication cycle. The specificity of an antiviral drug is the key to its success. These drugs are toxic to both the virus and the host but in order  to minimize their damage they are developed in such a way as to be more toxic to the virus than to the host. The efficiency of an antiviral drug is measured by the chemotherapeutic index, given by: