A trick used by viruses to make us sick - "stealth spheres" - has been discovered by scientists.

It had been thought viruses were all lone wolves, each on a solo campaign of infection.

Instead they can form "packs" of up to 40 viruses and surround themselves with a fatty sphere that makes them invisible to our body's defences.

The team at the US National Institutes of Health say their findings rewrite the textbooks of infection.

The researchers were analysing stool samples from patients with either rotavirus or norovirus.

Rotavirus is the biggest cause of diarrhoea in children in the world

Norovirus is so infectious it spreads rampantly through schools, care homes and frequently cruise ships
"It blew my mind," said researcher Dr Nihal Altan-Bonnet, the head of host-pathogen dynamics at the National Heart, Lung and Blood Institute.

"I had this preconceived notion... I couldn't believe it when I saw it in the stool, I just couldn't believe they'd be able to exist."

The conventional view is that a virus infects a cell, which is converted into a factory for making more viruses, which burst out as individual viruses into the body.

Instead some are released as groups in spheres known as vesicles.
Experiments, published in the journal Cell Host and Microbe, showed the spheres gave the viruses huge advantages.

The spheres acted like an invisibility cloak that prevents the immune system spotting the viruses
The spheres protected the viruses from harsh environments including stomach acid
And they helped viruses reach their target in larger numbers and overwhelm cells in the intestines
The researchers infected animals with a set number of viruses that were either released on their own or packaged into the stealth spheres.

Dr Altan-Bonnet told the BBC: "The animals that got the vesicles got disease much faster and had more severe symptoms.

"We think by being inside [the vesicles] the viruses become invisible and take longer to clear.

"In the viral world it's always about having an edge and in a vesicle you have a leg up in the early stages of the infection."

New therapies?
There are no drug treatments for either norovirus or rotavirus so the researchers hope targeting the vesicles will lead to new options.

That could be a type of handwash that breaks down the spheres or drugs that stop them forming in the first place.

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Dr Altan-Bonnet says the need for better ways of tackling diseases like norovirus should not be underestimated: "Even healthy people like you or I, we get so sick we wouldn't go to work for days.

"Its liquid coming violently out of both ends, there's a lot of economic cost, and for children and elderly it can be fatal."

The researchers say poliovirus and rhinovirus, a cause of the common cold, can also form these stealth spheres.

Jonathan Ball, a professor of virology at the University of Nottingham, said the findings were a surprise.

He told the BBC: "These vesicles seems to give the viruses increased infectivity when compared to naked individual viruses.

"Why this increased concentration of viruses results in better levels of entry is unclear, and no doubt will be an important next question.

"But what this data does indicate is how little we know about some of the most fundamental aspects of the virus lifecycle."