Viruses can spread faster than thought possible by surfing from cell to healthy cell while skipping cells that are already infected, scientists have discovered.

Unlike bacteria, viruses don't contain all the machinery necessary to replicate, and so like parasites they borrow the goods from other cells. Viruses were thought to spread by entering a cell, replicating there, and then being released to infect new cells. The rate of spread of a virus would therefore be limited by how quickly it could replicate in each cell.

However, a virus called vaccinia spreads four times faster than what was thought possible.

The virus bounces past cells that are already infected to more quickly reach uninfected cells, new microscopic videos of the cells revealed.

"This fundamentally changes how we think about virus dissemination," said lead researcher professor Geoffrey L. Smith from the Imperial College London. "Understanding how viruses spread is fundamental to designing strategies to block spread and thereby prevent disease."

Just after vaccinia infects a cell, it expresses two viral proteins on the cell surface, which marks the cell as infected. When further virus particles reach the infected cell, these proteins cause the host cell to push out snake-like projections called "actin tails," which drive the virus particles away toward other cells that they can infect. The particles thus bounce from one cell surface to another until they land on an uninfected cell.

In the study, the researchers prevented the vaccinia virus from making the proteins needed to make the actin tails in the early stages of cell infection and showed that this slowed the spread of the virus dramatically. Under normal conditions, vaccinia spread across one cell every 1.2 hours, which was slowed to one cell every five to six hours.

The discovery may ultimately enable scientists to create new antiviral drugs that target this newfound spreading mechanism. Other viruses, such as the herpes simplex virus that creates cold sores, may use the same mechanism, the scientists figure.

The work, funded by the Medical Research Council, is detailed in the Jan. 22 issue of the journal Science.