The Centers for Disease Control says HEPA filters are effective in clearing the air of many particles that cause disease. 疾病控制中心认为高效微粒空气过滤器能有效的清理掉空气中很多可致病的微粒。
With such a variety of factors,it is no wonder that scientists don't fully understand the natural history of the disease. 纵观以上各种不同的因素，无怪科学家们还未完全弄清致病的天然历史。
