Monitoring specific string or keyword in rotating log files.

Hi there,
I like to ask how i shall monitor specific string or keyword in rotating log files. e.g. I have at 10 rotating logfiles.
I use the command below to grep the string, but eventually become non functional because the logfile rotates and new logfile is active.

tail -f <logfile1> |grep <string>

Can you help how to resolve this?

Thank you.

There is a package called swatch which has plenty of features for such purposes.
Just google for "swatch log file monitoring" for download, guides etc.