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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?

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    "Only when someone dies has murder occurred" - credible. "Only when someone dies occurred murder" - not credible (except in Yoda speak).
    – FumbleFingers
    Commented Jul 28, 2012 at 4:10

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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

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  • Small question... isn't "have you a towel?" acceptable? What is the difference between this and "like you strawberries?" Is is applicable only for the verb 'have'?
    – notablytipsy
    Commented Jul 28, 2012 at 7:18
  • Have is a special case with "a foot in both camps" as I said with a very similar question
    – Andrew Leach ♦
    Commented Jul 28, 2012 at 10:02
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    @asymptotically I’d say “Have you a towel?” is at best marginally acceptable. It’s grammatical, but nobody ever says that. They say either “Do you have a towel?” or perhaps more commonly, “Have you got a towel?”, sometimes shortened to simply “Got a towel?”.
    – tchrist ♦
    Commented Jul 28, 2012 at 11:59
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    "Have you a towel?" is grammatical, but at least one grammar textbook says it's "less usual." The same with "I haven't a towel."
    – Cool Elf
    Commented Jul 28, 2012 at 12:03
  • No problem. The source I got that from is British btw
    – Cool Elf
    Commented Jul 28, 2012 at 12:10
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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".

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