From the text we learn that__________.阅读材料，回答下面的题目。 The Public Health System One of the biggest changes since 1990 is the degree to which bioterrorism has become a public health priority.Although there had long been concern about vulnerability to biowarfare and bioterrorism the anthrax episode in the fall of 2001 made it clear that the concern is no longer theoretical.Until very recently, the important role of public health at the frontlines of bioterrorism preparedness was unrecognized.Although concern about emerging infections has helped stimulate funding for the chronically under-appreciated public health system, the threat of bioterrorism motivated the first real infusion of new money into public health indecades.Many of the capabilities needed to defend against bioterrorism are the same: as those needed to combat natural emerging infections.In both instance, the problem is an unexpected outbreak of infectious disease, of which the first indication is likely to be sick people in emergency rooms or clinics.Indeed, as with the anthrax attacks, the public health and medical responses may be under way before the true nature of the outbreak is recognized. Public health and the interface with the health care system are therefore key elements in any effective response to bioterrorism.Whether the biggest threat is natural or engineered much remains to be done.Efforts to strengthen surveillance and response worldwide and to improve communication must be accelerated and sustained.Further,we have only scratched the surface in terms of understanding the ecology of infectious diseases and developing strategies for regulating microbial traffic.We need tools for better predictive epidemiologic modeling when a new infection first appears and for better analysis of the factors that transfer pathogens across species.One encouraging development is the program in the ecology of infections diseases that was started a few years ago be the National Science Foundation in cooperation with NIH.SARS is a good yardstick of our progress during the past 13 years.The syndrome was unusual because novel infections that spread from person to person are relatively rare.Once cases were finally reported, the public health response was vigorous.WHO warned health care providers, researchers rapidly identified a candidate virus, and prototype diagnostic tests quickly became available.The vast reach of the Internet was instrumental in sharing formation and coordinating activities worldwide.Despite these advances, SARS had already spread to many countries.In fact had the disease been as transmissible as influenza, it would have invaded virtually every country in the world by the time the public health response had begun.So what SARS tells us is that although we have come long way since 1990, we still have a long way to go.The main idea expressed in the first paragraph is that__________.

A. bioterrorism is an engineered threat to human beings
B. bioterrorism can be arrested by improving communication
C. natural infections and bioterrorism are the same in essence
D. the anthrax attacks are more threatening than the outbreak of SARS