What should I do when I do not get a reply to an email? I have sent out emails enquiring about academic positions in departments active in my field. I did my best to write them properly, consulting websites and more experienced academics.

However, it has been a week since I sent them, and have not heard back. 

Should I do something? Or wait? 

Clarifications:
I am in Zoology, and have enquired about potential applications.
This differs hugely by field and country, but in mine you don't contact the professors directly. If positions are available, then they will be advertised. Now you need inside/national knowledge where these positions will be advertised, but typically they will also appear on the website besides the "normal" venue. 

Imagine how many people are looking for a job. A prof would not be able to do her job if she had to answer all the job enquiries, so they are typically just ignored. This is not very nice, bit an understandable strategy of dealing with too many e-mails.