The potentially-fatal hepatitis C virus evades the body's natural defences by slipping directly from cell to cell, scientists have found.
Thousands are infected with hepatitis C
This could mean treatments aimed at interrupting its progress won't work as well as hoped.
University of Birmingham researchers told a Glasgow conference that this could explain the rapid spread of the virus in some patients.
The British Liver Trust said that a treatment was "desperately needed".
Viruses spread by entering cells, then replicating themselves, with large numbers of copies of the virus bursting out of the cell to start the process again.
However, some viruses don't have to leave the host cell before infecting another - they can move directly between cells.
It had been thought that Hepatitis C didn't have this ability, but the Birmingham research, using liver tumour cells infected with the virus, now suggests that it does.
"Cell to cell transmission" allows the virus to bypass some of the body's most potent defence systems - antibodies can only attack outside the cell.
This will be a disappointment for scientists who hoped to boost antibody defences in order to stop the virus in its tracks.
Dr Jennifer Timpe, who presented the research to the International Symposium on Hepatitis C Virus in Glasgow, said: "This is probably why it has been so tricky to tackle.
"Finding that Hep C uses multiple mechanisms for spreading around the body was not great news, but this discovery will allow those of us working in this area to move ahead with a better understanding of the virus.
"We will have to up our game and find other ways of tackling this relentless virus."
A spokesman for the British Liver Trust said: "Gaining a better understanding of the virus will certainly go some way into the development of treatment for hepatitis C patients - something which is desperately needed.
"At the moment approximately one in five people with a chronic infection of hepatitis C develop severe liver damage which can lead to liver cancer or liver failure and the need for liver transplantation.
"These results indicate to a positive result for treatment solutions."