Australia's koala population has been hit hard by two rapidly spreading diseases: chlamydia (a sexually transmitted infection) and a retrovirus similar to HIV. Scientists are working to develop vaccines, while lay citizens help care for sick koalas. Biologists say the epidemics, combined with other threats like habitat loss, pose a serious threat to the species.For more global science stories, visit: http://www.world-science.org/Funding for NOVA is provided by David H. Koch, the Corporation for Public Broadcasting, and PBS viewers.