nozzle of the jet injector is sucked back into the injector orifice, contaminating the next dose to be fired. The CDC has acknowledged that the most widely used jet injector in the world, the Ped-O-Jet, sucked fluid back into the gun. "After injections, they [CDC] observed fluid remaining on the Ped-O-Jet nozzle being sucked back into the device upon its cocking and refilling for the next injection (beyond the reach of alcohol swabbing or acetone swabbing)," stated Dr. Bruce Weniger. Retrograde flow Retrograde flow happens after the jet stream penetrates the skin and creates a hole, if the pressure of the jet stream causes the spray, after mixing with tissue fluids and blood, to rebound back out of the hole, against the incoming jet stream and back into the nozzle orifice. This problem has been reported by numerous researchers. Hepatitis B can be transmitted by less than one nanolitre so makers of injectors must ensure there is no cross-contamination between applications. The World Health Organization no longer recommends jet injectors for vaccination due to risks of disease transmission. Numerous studies have found cross-infection of diseases from jet injections. An experiment using mice, published in 1985, showed that jet injectors would frequently transmit the viral infection lactate dehydrogenase elevating virus (LDV) from one mouse to another. Another study used the device on a calf, then tested the fluid remaining in the injector for blood. Every injector they tested had detectable blood in a quantity sufficient to pass on a virus such as hepatitis B. From 1984 to 1985, a weight-loss clinic in Los Angeles administered human chorionic gonadotropin (hCG) with a Med-E-Jet injector. A CDC investigation found 57 out of 239 people who had received the jet injection tested positive for hepatitis B. Jet injectors have also been found to inoculate bacteria from the environment into users. In 1988 a podiatry clinic used a jet injector to deliver local anaesthetic into patients' toes. Eight of these patients developed infections caused by Mycobacterium chelonae. The injector was stored in a container of water and disinfectant between use, but the organism grew in the container. This species of bacteria is sometimes found in tap water, and had been previously associated with infections from jet injectors. History 19th century: Workmen in France had accidental jet injections with high-powered grease guns. December 18, 1866: Jules-Auguste Béclard presented Dr. Jean Sales-Girons invention, Appareil pour l'aquapuncture to