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The origin of viruses remains mysterious because of their diverse and patchy molecular and functional makeup. Although numerous hypotheses have attempted to explain viral origins, none is backed by substantive data. We take full advantage of the wealth of available protein structural and functional data to explore the evolution of the proteomic makeup of thousands of cells and viruses. Despite the extremely reduced nature of viral proteomes, we established an ancient origin of the "viral supergroup" and the existence of widespread episodes of horizontal transfer of genetic information. Viruses harboring different replicon types and infecting distantly related hosts shared many metabolic and informational protein structural domains of ancient origin that were also widespread in cellular proteomes. Phylogenomic analysis uncovered a universal tree of life and revealed that modern viruses reduced from multiple ancient cells that harbored segmented RNA genomes and coexisted with the ancestors of modern cells. The model for the origin and evolution of viruses and cells is backed by strong genomic and structural evidence and can be reconciled with existing models of viral evolution if one considers viruses to have originated from ancient cells and not from modern counterparts.

Questions about the nature of viruses remain quite vexing. Recent studies of the giant Mimivirus illustrate this point. Its large size and correspondingly large genome test our general ideas of viruses as small, simple entities. The existence of genes associated with translation, metabolism, DNA repair, and protein folding raises questions about the evolutionary history of viruses. Further studies of this virus, and the search for other giant viruses, may shed light on these issues.

Avian influenza refers to the disease caused by infection with avian (bird) influenza (flu) Type A viruses. These viruses occur naturally among wild aquatic birds worldwide and can infect domestic poultry and other bird and animal species. Avian flu viruses do not normally infect humans. However, sporadic human infections with avian flu viruses have occurred. The links below offer information about avian influenza for different audiences.

What does it mean to be 'alive'? At a basic level, viruses are proteins and genetic material that survive and replicate within their environment, inside another life form. In the absence of their host, viruses are unable to replicate and many are unable to survive for long in the extracellular environment. Therefore, if they cannot survive independently, can they be defined as being 'alive'?

Scientists are not sure whether viruses are living or non-living. In general, scientists use a list of criteria to determine if something is alive. Let's look at some traits of living things and see if viruses also have those traits.

Viruses have a huge impact on our lives, and we're making great strides into understanding how to protect ourselves from the flu and HIV. But one thing that scientists have struggled to agree on is whether or not viruses are alive.

A paper published today in Science Advances just might change that. By creating a reliable method of studying viruses' long evolutionary history-hitherto nearly impossible-researchers have found new evidence that strongly suggests viruses are indeed living entities.

Last month, researchers announced they were studying a 30,000-year-old giant virus called Mollivirus sibericum that they found in melted Siberian permafrost. The virus was functional and able to infect amoeba. This isn't the first time researchers have found big viruses that have challenged what we thought we knew about the tiny invaders. Mimivirus, discovered in 2003, has 1,200 genes and is twice the width of traditional viruses.

Around the world, disease-carrying mosquitoes are advancing at speed, taking viruses such as dengue and Zika, plus a host of lesser-known ills such as chikungunya and St. Louis encephalitis, into new territories from Europe to the Pacific. "The concern is that we have these species spreading everywhere. Today the focus is on Zika but they can carry many different viruses and pathogens," said Anna-Bella Failloux, head of the department that tracks mosquito viruses at France's Institut Pasteur.

In recent years, Claverie and his colleagues have discovered a host of giant viruses, which are as big as bacteria but lack characteristic cellular machinery and metabolism of those microorganisms. At least one family of these viruses likely evolved from single-celled parasites after losing essential genes, although the origins of other giant viruses remain a mystery, Claverie said.

How did viruses evolve? Are they a streamlined form of something that existed long ago, or an ultimate culmination of smaller genetic elements joined together?

If thinking about the billions of bacteria taking up residence in and on your body gives you the willies, you probably won't find it comforting that humans are also full of viruses. These maligned microbes are actually intertwined in the very fibers of our being-about 8 percent of our genetic material is made up of absorbed forms of retroviruses, the viral family to which HIV, the pathogen that causes AIDS, belongs.

Our genomes are riddled with the detritus of ancient viruses. They infected our hominid ancestors tens of millions of years ago, inserting their genes into the DNA of their hosts. Today, we carry about 100,000 genetic remnants of this invasion. So-called endogenous retroviruses make up 8 percent of the human genome.

Can't remember every viral infection you've ever had? Don't worry, your blood can. A new test surveys the antibodies present in a person's bloodstream to reveal a history of the viruses they've been infected with throughout their life. The method could be useful not only for diagnosing current and past illnesses, but for developing vaccines and studying links between viruses and chronic disease.

Are viruses life forms or not? Scientists have been fighting about it for years, and Anthony has more on the latest wrinkle in this seemingly simple yet age-old debate.

The test, dubbed VirScan, and research surrounding it is the subject of a new report published in the journal Science. For the study, scientists screened sera, a part of the blood, from 569 people in the U.S., South Africa, Thailand, and Peru and tested for more than 200 types of viruses.

The table below displays the list of human viral pathogens, with transmission and general facts about associated pathologies.

Which pathogens play a major role in causing infection? Just recently, the Robert Koch-Institute prepared a list of 127 pathogens by ranking them in terms of frequency, spread and burden of disease.   Considering this prioritisation, we extended our search tool by bacteria, viruses and fungi accordingly. In the list, you will find both the clinically relevant pathogens and the spectrum of effect necessary for disinfectants to inactivate them.

Throughout the last few decades, diseases that spill over from animals to humans have been on the rise. What's behind their increase, and can we do anything to combat these dangerous foes? Join scientists as they investigate the rise of spillover viruses like Zika, Ebola and Nipah, and learn what science can do to anticipate and prevent epidemics around the world.

Multiple outbreaks have been reported in poultry farms and wild flocks across Europe, Africa and Asia in the past three months. While most involve strains that are currently low risk for human health, the sheer number of different types, and their presence in so many parts of the world at the same time, increases the risk of viruses mixing and mutating - and possibly jumping to people.