Biological viruses

In general, cells reproduce by making a copy of their DNA. Unlike cells, viruses do not have the tools to make a copy of their DNA. But they have found other ways to make new viruses. This is done by inserting virus genetic material into a host cell. This causes the cell to make a copy of the virus DNA, making more viruses.

Many scientists argue that even though viruses can use other cells to reproduce itself, viruses are still not considered alive under this category. This is because viruses do not have the tools to replicate their genetic material themselves. More recently, scientists have discovered a new type of virus, called a mimivirus.

These viruses do contain the tools for making a copy of its DNA. This suggests that certain types of viruses may actually be living. Viruses only become active when they come into contact with a host cell.

Image by CarlosRoBe. Living things use energy. Outside of a host cell, viruses do not use any energy. They only become active when they come into contact with a host cell. Because they do not use their own energy, some scientists do not consider them alive. This is a bit of an odd distinction though, because some bacteria rely on energy from their host, and yet they are considered alive. These types of bacteria are called obligate intracellular parasites. The first line of defence against emerging viruses is effective surveillance.

This topic has been widely discussed in recent years [ 10 , 41 ], but we will re-iterate a few key points here. Firstly, emerging viruses are everyone's problem: the ease with which viruses can disperse, potentially worldwide within days, coupled with the very wide geographical distribution of emergence events [ 9 ], means that a coordinated, global surveillance network is essential if we are to ensure rapid detection of novel viruses.

This immediately highlights the enormous national and regional differences in detection capacity, with the vast majority of suitable facilities located in Europe or North America. Secondly, reporting of unusual disease events is patchy, even once detected, reflecting both governance issues and lack of incentives [ 10 ]. Thirdly, we need to consider extending the surveillance effort to other mammal populations as well as humans, because these are the most likely source of new human viruses.

Improving the situation will require both political will and considerable investment in infrastructure, human capacity and new tools [ 10 , 41 ]. However, the benefits are potentially enormous. It is possible to forestall an emerging disease event, as experience with SARS has shown.

However, our ability to achieve this is closely linked to our ability to detect such an event, and deliver effective interventions, as rapidly as possible. A better understanding of the emergence of new human viruses as a biological and ecological process will allow us to refine our currently very crude notions of the kinds of pathogens, or the kinds of circumstances, we should be most concerned about, and so direct our efforts at detection and prevention more efficiently.

We are grateful to colleagues in Edinburgh's Epidemiology Research Group and elsewhere for stimulating discussions and to two anonymous referees for thoughtful comments on the manuscript.

National Center for Biotechnology Information , U. Author information Copyright and License information Disclaimer. This is an open-access article distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. This article has been cited by other articles in PMC.

Abstract There are virus species that are known to be able to infect humans. Keywords: discovery curves, emerging infectious diseases, public health, risk factors, surveillance. Virus diversity and discovery a Survey of human viruses As a starting point for our survey, we used a previously published database see [ 5 ] obtained by systematically searching the primary scientific literature up to and including for reports of human infection with recognized virus species, using species as defined by the International Committee on Taxonomy of Viruses ICTV [ 6 ].

Open in a separate window. Figure 1. Table 1. Figure 2. Table 2. Emergence as a biological process a Non-human reservoirs More than two-thirds of human virus species are zoonotic, i. Figure 3. Figure 4. Conclusions The lines of evidence described earlier combine to suggest the following tentative model of the emergence process for novel human viruses. References 1. Levine A. History of virology.

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Identification of a third human polyomavirus. Chua K. A previously unknown reovirus of bat origin is associated with an acute respiratory disease in humans. USA , 11 —11 Gaynor A. Villarreal and DeFilippis and Bell described models explaining this proposal. Perhaps, both groups postulate, the current nucleus in eukaryotic cells arose from an endosymbiotic-like event in which a complex, enveloped DNA virus became a permanent resident of an emerging eukaryotic cell.

Where viruses came from is not a simple question to answer. One can argue quite convincingly that certain viruses, such as the retroviruses, arose through a progressive process. Mobile genetic elements gained the ability to travel between cells, becoming infectious agents.

One can also argue that large DNA viruses arose through a regressive process whereby once-independent entities lost key genes over time and adopted a parasitic replication strategy. Finally, the idea that viruses gave rise to life as we know it presents very intriguing possibilities. Perhaps today's viruses arose multiple times, via multiple mechanisms. Perhaps all viruses arose via a mechanism yet to be uncovered.

Today's basic research in fields like microbiology, genomics , and structural biology may provide us with answers to this basic question. Contemplating the origins of life fascinates both scientists and the general public. Understanding the evolutionary history of viruses may shed some light on this interesting topic.

To date, no clear explanation for the origin s of viruses exists. Viruses may have arisen from mobile genetic elements that gained the ability to move between cells. They may be descendants of previously free-living organisms that adapted a parasitic replication strategy. Perhaps viruses existed before, and led to the evolution of, cellular life. Continuing studies may provide us with clearer answers. Or future studies may reveal that the answer is even murkier than it now appears.

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It is a type of viral hemorrhagic fever VHF brought on by any of several strains of viruses in the Ebolavirus genus. Ebola viruses are capable of causing severe, life-threatening disease. Foodborne Disease. Foodborne illnesses are caused by viruses, bacteria, parasites, toxins, metals, and prions microscopic protein particles.

Symptoms range from mild gastroenteritis to life-threatening neurologic, hepatic, and renal syndromes. Hantaviruses are transmitted to humans from the dried droppings, urine, or saliva of mice and rats. Animal laboratory workers and persons working in infested buildings are at increased risk to this disease. Legionnaires' Disease. Legionnaires' disease is a bacterial disease commonly associated with water-based aerosols. It is often the result of poorly maintained air conditioning cooling towers and potable water systems.

Measles is a vaccine-preventable but highly contagious and potentially serious bacterial disease that was previously eliminated in the United States. Unvaccinated travelers often bring the disease back from abroad, spreading it to other susceptible people and causing periodic outbreaks.

MERS is a potentially fatal, emerging respiratory disease caused by a novel coronavirus that primarily affects the lungs and breathing passages.