Viruses are submicroscopic particles of genetic material contained in an envelope, parasites that live at the expenses of the host organism and reproduce using the host’s genetic material to duplicate their DNA or RNA, are ubiquitous in the ecosystem, are found everywhere in millions of species.
Based on the infected host, the viruses are classified into animal, plant and fungal viruses, and bacteriophages that are viruses that infect bacteria and are the most complex; today are used in probiotics formula to function as antibiotics.
Other classifications use the geometric shape of the capsid, (the envelope that encloses the genetic material and which usually has the form of an icosahedron, or the presence or not of a lipidic envelope.
Outside the cell they exist as independent units called virions and which consist of genetic material, DNA or RNA that encodes for the structure of proteins with which viruses penetrate the host cells, the capsid or coat, and the lipidic envelope if present.
Viruses are measured in nanometers corresponding to one thousandth of a micron which in turn corresponds to the thousandth of a millimeter and for this reason the smallest can be observed only at the electronic microscope.
The most widely used classification distinguishes viruses based on the nucleic acid that makes up the genetic material and the replication method used in the host cell
This classification distinguishes:
- DNA viruses divided into dual-chain and single-chain viruses
- RNA viruses divided into single-chain positives and single-chain negatives and less common RNA double-chain
- Virus with double chain of DNA reverse transcriptase and single chain of RNA reverse transcriptase to which the Retroviruses, the viruses responsible for HIV/AIDS belong.
Reverse Transcriptase are enzymes that allow the formation of DNA from a template of RNA, they are named “reverse” because normally the transcription of RNA is made from a stamp of DNA.
Viruses appear to have a short generation time, and RNA viruses, as SARS-CoV-2, responsible for COVID-19 is, in particular have elevated mutation rates, which when combined with natural selection allows the virus to adapt to host environmental changes and pass the characteristic to subsequent generations.
The origins of viruses are unclear: some may have evolved from plasmids—pieces of DNA that can move between cells—while others from bacteria. In evolution, viruses are an important means of horizontal gene transfer, which increases genetic diversity in a way analogous to sexual reproduction.
Viruses are considered by some biologists to be a life form because they carry genetic material, reproduce, and evolve through natural selection, even though they lack the key characteristics as cell structure that are generally considered necessary criteria for life.
Viral evolution is an important aspect of the epidemiology of viral disease, with the flu for example, the rapidity of the viral mutation causes problems in the development of vaccines and antiviral drugs, so as is happening with COVID-19.
Viral pathogenesis studies how viruses cause the disease, while virulence is the ability to cause the disease.
Viruses spread in many ways. One transmission pathway is through vectors: for example, viruses in animals can be transferred by blood sucking insects as mosquitos. Influenza viruses are spread by coughing and sneezing. Norovirus and Rotavirus, common causes of viral gastroenteritis, are transmitted by the fecal-oral route, passed by hand-to-mouth contact or in food or water.
HIV is one of several viruses transmitted through sexual contact and by exposure to infected blood. The variety of host cells that a virus can infect is called “host range” This can be narrow, meaning a virus is capable of infecting few species, or broad, if it is capable of infecting many.
Viral infections in animals provoke an immune response that usually eliminates the infecting virus. Immune responses can also be produced by vaccines, which confer an artificially acquired immunity to the specific viral infection. Some viruses, including those that cause AIDS, HPV, and viral Hepatitis, evade these immune responses and result in chronic infections. Several antiviral drugs have been developed against these.
When the immune system encounters a virus it produces specific antibodies that bind to the virus by neutralizing the infection or present it to other cells of the immune system for destruction, these are called T-cells, or T-lymphocytes, antibodies are instead produced by B-lymphocytes, both cells, or lymphocytes B and T are part of the white cells, or white blood cells of the immune system.
T-cells are responsible for the production of molecules called cytokines such as interleukins and interferon, these together with macrophages and platelets and other molecules are part of our body’s complex defense system against all sorts of external attacks.
Vaccines must consider this type of immune activation into account to avoid immunological reactions. The lethal diseases appear to have resulted from accidents (zoonosis) due to jumps from a host species where they were benign to another species and where instead they become lethal and cause of serious and dangerous diseases, such as the coronavirus responsible for COVID-19 for which the original source were bats and few other animals.
The development of a vaccine for SARS-CoV-2, COVID-19 related, is still undergoing because of the difficulties encountered with this virus and the type of immunity that it activates and at present antiviral drugs are among the most effective solutions yet to appear and work in many cases along with the use of antibiotics and alternative therapies that strengthen the immune system.
For info on how to support the immune system a previous blog of mine can provide specific information, this the link:
Other strategies are instead the application of passive immunity through the use of serums made of antibodies against the virus obtained from convalescent COVID-19 patients which seems to work pretty well in many cases, and ultimately approved by the governmental health agencies as one of the therapies of choice, as much as they were doubting the efficacy of this therapy because of the antibodies time frame of duration.
To be continued.
Thanks for Reading
Source and Rearranging: “Virus”, Wikipedia
Picture: Bacteriophage, by National Geographic.com