（which is）so rare that…

我要是老师，看到学生写字能写得和印刷版乱真的程度，我才不管你的语法啥的，直接给高分。卷面分和态度分爆满

没看懂你啥意思，你是认为老师看不懂？还是不确定你写的是对是错

这句话啥意思，我只看懂前半句，后面是八百万分之一的意思吗

我看着没错，老师水平没你想象的那么低 就一个常规的简单的省略句 肯定看得出来，

句子很好，没问题

正常。

我觉得没毛病

这句子也不算太高级吧，so后面部分都是用来讲这个病的

just生硬，这样更好doctors diagnosed him with a disease so rare that only 1 in 8 million is affected.
only换成merely或者but也行。
doctors diagnosed him with a rare disease which affects only 1 in 8 million. 也行。
doctors diagnosed him with a disease rare enough that it's seen 1 in 8 million. 也行。
doctors diagnosed him with a rare disease with which only 1 in 8 million is diagnosed. 也行。
doctors diagnosed him with a rare disease, affecting but 1 in 8 million. 也行。
要多变，老师应该不会因为你复杂而判错。

没毛病