Inversion in "Only when the virus introduces its nucleic acid into a cell does disease occur"

Given this sentence,

Disease occurs only when the virus introduces its nucleic acid into a cell.

Is the following inversion grammatical?

→ Only when the virus introduces its nucleic acid into a cell does disease occur.

Especially the "does disease occur" part.

I think that "occurs disease " is right. Can you explain a rule about this kind of sentence?

Solution 1:

Your sentence is right.

A simple way of looking at the rule of inversion is thinking about question and sentence patterns in English.

The question pattern is:

Verb (V) + Subject (S) 

Some learners actually misunderstand the meaning of "v." In the same way that these questions are wrong and unacceptable:

Went he to school?

Like you strawberries?

Your alternative is also wrong:

Occurs disease?

I pointed out these things because the pattern for inversion is the same pattern for questions. And this pattern is:

Auxiliary Verb + Subject + Main Verb 

So,

Only when... does disease occur. = OK

Solution 2:

There are lots of different cases where inversion is permissible. There's a really good list at http://esl.about.com/od/advancedgrammar/a/inversion.htm. There are sentences with the structure that you've exhibited, using "only", on this list. There's more detail available about this particular case at http://www.bbc.co.uk/worldservice/learningenglish/grammar/learnit/learnitv34.shtml.

The key point with inversion is that there are are only a limited number of verbs that can come before the subject when you use inversion - just modal verbs and auxiliary verbs. So, you can invert "does" (that is, put it before "disease", as per your example). But you can't invert "occurs".