migration patterns of water bird species. It is therefore of great importance to know the ecological strategy of influenza virus as well, in order to fully understand this disease and to control outbreaks when they occur. Most research is needed concerning HPAI viruses in wild birds. For example, small birds like sparrows and starlings can be infected with deadly H5N1 strains and they can carry the virus from chicken house to chicken house causing massive epidemics among the chickens. However, pigeons do not present a risk as they neither catch nor carry the virus. Avian flu in humans Human to human transmission The WHO believes that another influenza pandemic is as likely to occur at any time since 1968, when the last century's third of three pandemics took place. The WHO describes a series of six phases, starting with the inter-pandemic period, where there are no new influenza virus subtypes detected in humans, and progressing numerically to the pandemic period, where there is efficient and sustained human-to-human transmission of the virus in the general population. At the present moment, we are at phase 3 on the scale, meaning a new influenza virus subtype is causing disease in humans, but is not yet spreading efficiently and sustainably among humans. So far, H5N1 infections in humans are attributed to bird-to-human transmission of the virus in most cases. Until May 2006, the WHO estimate of the number of human to human transmission had been "two or three cases". On May 24, 2006, Dr. Julie L. Gerberding, director of the United States Centers for Disease Control and Prevention in Atlanta, estimated that there had been "at least three." On May 30, Maria Cheng, a WHO spokeswoman, said there were "probably about half a dozen," but that no one "has got a solid number." A few isolated cases of suspected human to human transmission exist. with the latest such case in June 2006 (among members of a family in Sumatra). No pandemic strain of H5N1 has yet been found. Prevention Notwithstanding possible mutation of the virus, the probability of a "humanized" form of H5N1 emerging through genetic recombination in the body of a human co-infected with H5N1 and another influenza virus type (a process called reassortment) could be reduced by widespread seasonal influenza vaccination in the general population. It is not clear at this point whether vaccine production and immunization could be stepped up sufficiently