What can we know about viruses and bacteria from the fourth paragraph?

细节题。根据原文第四段中的“The crucial bit ofthe puzzle is that viruses have to becaught from another infected individual．．．Dr．Cole thus suggests that past evolution has created a mechanism whichcauses white cells to respond appropriately”可知，病毒通过另一个已感染此病毒的人传染给某个人，并且病毒通常有其一对一的特殊宿主。相比之下，细菌经常仅仅潜伏在周围的环境中，并且宿主众多。所以爱交际的人比孤独的人更易感染病毒。由此可见，病毒较之于细菌对爱交际的人的威胁更大，故本题选D。原文提到经常潜伏在周围环境中的是细菌，而不是病毒，故A项错误。B项描述的是病毒的传染方式。C项与原文中的“The gregarious are therefore at greater risk than the lonely ofcatching viruses”描述矛盾。