【英语时差8,16】抗体如何预防艾滋病？

 One way our bodies resist disease is by producing a variety of chemicals called “antibodies,” which destroy the virus or bacterium¹ causing the disease. Antibodies have to be selective to avoid destroying the wrong cells, so each type of antibody recognizes and attacks only a specific virus or bacterium. And that means that each new disease requires a new type of antibody. But in fighting disease, evolution sometimes works against us because the faster an organism multiplies, the faster it adapts to changes in its environment. For bacteria, the appearance of a new antibody means a big change in its environment, but unfortunately it’s a change the bacteria often adapt to. Since a single bacterium can multiply a thousand-fold in three hours, there’s a good chance that some of those offspring will vary from the rest in ways that make them unrecognizable to the antibody. Those bacteria will then produce millions more like themselves.

1 bacterium   

n.(pl.)bacteria 细菌

参考例句：

• The bacterium possibly goes in the human body by the mouth.细菌可能通过口进入人体。
• A bacterium is identified as the cause for his duodenal ulcer.一种细菌被断定为造成他十二指肠溃疡的根源。