Due to the progress in the field of modern medicine and advances in nutrition and hygiene, less people die every year from infectious diseases then ever before.

The highlighted portion in the question has a couple of issues. The first is the use of the word 'then' as opposed to 'than', which should be used in comparative contexts like the one presented in the question. Another issue is, the use of the word 'less', when used with the word 'than', the word should be in comparative form, i.e. 'lesser'. And, since 'people' is not a number, we'll use the word 'fewer' instead of 'lesser'. The fixed portion should be "fewer people die every year from infectious diseases than ever before." Options A and B are incorrect as they use 'then'. Option D is also incorrect as it reads 'than have ever before' instead of 'than ever before', which misquotes the original. Option C is the correct choice.