表面上看这个if从句是个条件状语从句，却位于表语的位置。但你仔细读这个句子，你就可以知道表语从句其实是个主从复合句，主句it may do so省略了。

For example, one explanation for why a cell may deliberately destroy itself is (that) (it may do so) if its sacrifice lowers the viral load on closely related cells nearby.

这个句子稍作修改，则表语为主从复合句就更明显了：

Why will a cell deliberately destroy itself? One explanation is （that）it will do so if its sacrifice lowers the viral load on closely related cells nearby.

这个句子中it will do so就不可以省略了。