Dr. Mario Express
This game is titled Dr. Mario Express in North America and Chotto Dr. Mario in Japan.
If you set the virus level to 20 and complete it, you will start at the next level, level 21. The levels will continue increasing, however when you get to level 35 it will reach the maximum number of viruses, leaving only two free rows of space to drop the pills. Note: Even after you complete the higher levels, you must re-access them by completing level 20, then level 21, etc.