A virus comprises a thin shell of protein, within which are its genes. A long-standing theory has been that a virus has high internal pressure because it is so tightly packed with genetic material.
The pressure means that they can infect a cell by ejecting the genes at high force and speed.
The cell is then duped into becoming a small 'virus factory' that produces new viruses, multiplying the number.
Biochemist Alex Evilevitch from Lund University and Carnegie Mellon University in Pittsburgh, USA, measured the pressure inside the herpes virus HSV-1 (herpes simplex virus 1) together with a research team in the US.
This includes both of the two most common forms of herpes, which cause cold sores and genital herpes, as well as Varicella zostervirus, which causes chickenpox and shingles, Epstein-Barr virus, which leads to glandular fever, and viruses linked to various forms of cancer.
The study has been published in the Journal of the American Chemical Society, JACS.