The latest flu virus causing concern, H7N9, arose in birds in Eastern China a few months ago—so far infecting more than 100 people, with a high death rate . To gauge the pandemic potential of this new avian virus, a team of Chinese and NIH-funded American researchers isolated the virus from a patient in China and used it to infect ferrets .
Yes, you read that right: ferrets! It turns out that ferret airways have biological similarity to humans, and so they are traditionally used as an indicator of whether humans are susceptible to a particular flu virus and whether transmission can occurs through the air (breathing, coughing, or sneezing) or requires direct contact.
For H7N9, the researchers infected a group of six ferrets (using a drop of virus in the nostril) and then put three uninfected ferrets in separate cages nearby; three more uninfected animals were put directly into the cages with infected ferrets. It seems that ferrets can transmit H7N9 virus quite efficiently via direct contact; all ferrets in direct contact became infected. But only one of three neighboring animals contracted the virus through the air. The ferrets were also able to transmit the virus before they showed signs of infection (coughing, sneezing, runny nose).
The more easily a flu virus spreads through the air, the greater the potential for a pandemic. While at present, H7N9 seems to be quite limited in this ability, the virus could evolve to become more transmissible. That’s why CDC and NIH are watching this one closely.
 Number of confirmed human cases of avian influenza A(H7N9) reported to WHO. (World Health Organization, Report 6, May 16, 2013)
 Infectivity, Transmission, and Pathology of Human H7N9 Influenza in Ferrets and Pigs. Zhu H, Wang D, Kelvin DJ, Li L, Zheng Z, Yoon SW, Wong SS, Farooqui A, Wang J, Banner D, Chen R, Zheng R, Zhou J, Zhang Y, Hong W, Dong W, Cai Q, Roehrl MH, Huang SS, Kelvin AA, Yao T, Zhou B, Chen X, Leung GM, Poon LL, Webster RG, Webby RJ, Peiris JS, Guan Y, Shu Y. Science. 2013 May 23.
NIH support: National Institute of Allergy and Infectious Diseases