资料题：Many people believe that viruses cannot possibly be of importance in human cancer because（  ）。

What is anthrax?Anthrax(炭疽热) is a colorless, odorless, tasteless bacterium that protects itself from sunlight, beat and disinfectant by forming a protective coat.With this coat, the bacterium is called a "spore.”The spares are so small that even an infectious dose - between 8,000 and 10,000spores - is smaller than a speck of dust.If inhaled, anthrax spores can germinate and lead to infection within one to60 days.This is pulmonary anthrax, which usually causes death.Less lethal forms include cutaneous infections,which occur if anthrax spores come in contact with a cut or other opening in the akin.Another form of anthrax develops in the intestine(肠道) of those who eat improperly cooked meat of animals that have been infected with anthrax.What are the symptoms of anthrax?Symptoms of pulmonary anthrax are very similar to the flu, which can make an initial diagnosis somewhat difficult.However, in light of recent events many doctors have a heightened awareness of the possibility, and are therefore more likely to diagnose it.Symptoms of the disease vary depending on how the disease was contracted, but usually occur within seven days.The Centers for Disease Control list the following:Cutaneous: Most (about 95 percent) anthrax infections occur when the bacterium enters a cut or abrasion on the akin, such as when handling contaminated wool, hides, leather or hair products (especially goat hair) of infected animals.Skin infection begins as a raised itchy bump that is dark in color and resembles an insect bite but within 1 - 2 days develops into a vesicle and then, a painless ulcer, usually 1- 3 centimeters in diameter, with a characteristic black area in the center.Lymph glands in the adjacent area may swell.About 20 percent of untreated cases of cutaneous anthrax will result in death.Deaths are rare with appropriate therapy .Inhalation: Initial symptoms may resemble a common cold.After several days, the symptoms may progress to severe breathing problems and shock.Inhalation anthrax is usually fatal.Intestinal: The intestinal disease form of anthrax may follow the consumption of contaminated meat and is characterized by am acute inflammation of the intestinal tract.Initial signs of nausea, loss of appetite, vomiting, fever are followed by abdominal pain, vomiting of blood, and severe diarrhea.,Intestinal anthrax results in death in 25 percent to 60 percent of cases.