How big is HIV?

HIV, the human immunodeficiency virus, is the virus that causes AIDS, acquired immunodeficiency syndrome. Its particles are roughly round and vary in size, from about 120 to 200 nanometers across, with an average of about 145 nanometers. A nanometer is a billionth of a meter.

Size

HIV particles do not all have the same size or shape. Each one is wrapped in a membrane, a thin skin made of fatty molecules. The size of a particle means its diameter, the distance across it.

The particles are measured with cryo-electron microscopy. Scientists freeze them very quickly and photograph them with an electron microscope. In a study of 255 mature HIV particles, the diameters ranged from 120 to 200 nanometers and averaged 145 nanometers.

A 2012 review of HIV's structure gives a somewhat smaller range, 100–150 nanometers. New particles do not change size as they mature into their infectious form.

Size comparisons

An HIV particle is about three and a half times as wide as a hepatitis B virus particle, which is 42 nanometers across.
About 50 HIV particles side by side would span a red blood cell, which is about 7.5 micrometers wide. A micrometer is a thousandth of a millimeter.
About 550–700 HIV particles in a row would span the width of a human hair, 80–100 micrometers.
An ordinary light microscope cannot show an HIV particle's shape. The finest detail it can show is about 200 nanometers, the limit of optical microscopes, and a normal-sized HIV particle is smaller than that.
If an HIV particle were enlarged to the size of a basketball, 24 centimeters (9.4 inches) across, a red blood cell would be about 12 meters (41 feet) across.

Inside an HIV particle

The outer membrane is studded with spikes of a protein called Env. The spikes grab onto a cell and join the virus's membrane to the cell's, which starts an infection. Within the membrane lies a cone-shaped shell, the capsid, built from about 1,500 copies of a single protein.

The capsid holds two copies of the virus's genes, written in single strands of RNA, a molecule related to DNA. Cones separated from HIV particles averaged 119 nanometers long and 60 nanometers across at their widest. Of the 255 particles measured for diameter, about a third held two or more cones, and particles with two were larger on average, 159 nanometers across compared with 134.

How HIV was discovered

Doctors first reported a new disease that weakened the immune system in 1981. The French virologists Françoise Barré-Sinoussi and Luc Montagnier grew cells from the swollen lymph nodes of patients. They found a new virus budding from the cells. It infected and killed lymphocytes, a kind of white blood cell that includes the T cells essential to the body's defenses.

Other groups soon showed that this virus, now called HIV, causes AIDS. Barré-Sinoussi and Montagnier were awarded half of the 2008 Nobel Prize in Physiology or Medicine for the discovery. The prize announcement noted that with successful treatment, people with HIV can expect to live about as long as people without it.