During the first North American outbreak of the disease, many of the deaths reported to have been caused by CXC were actually due to other causes. Since the first outbreak, farmers have learned to treat mild cases of CXC and no longer report them to veterinarians or authorities. An inoculation program against CXC was recently begun after controlled studies showed inoculation to be 70 percent effective in preventing serious cases of the illness. Many recent cattle deaths that have actually been caused by CXC have been mistakenly attributed to another disease that mimics the symptoms of CXC.
Five years ago, during the first North American outbreak of the cattle disease CXC, the death rate from the disease was 5 percent of all reported cases, whereas today the corresponding figure is over 18 percent. It is clear, ttherefore, that during these past 5 years, CXC has increased in virulence.
Which one of the following, if true, most substantially weakens the argument?