Combinations are integral to your success in this game, especially when playing with the many skilled players online -- To remove a virus from the board, you have to stack three like-colored blocks around it to form a straight line (whether horizontal or vertical). However, if you stack, say, two blocks on a virus and let it linger, and do the same to a virus right next to it, you're set up beautifully for a combination. Say you have a red and yellow virus adjacent to each other, both stacked two high on top of each virus. The next red-yellow pill that comes should go atop both of them to eliminate them at the same time. This is the most basic of combinations, and will send two random blocks onto your opponent's board, disrupting his efforts.
Virus combinations get more complex and more complicated than this, but remember that combinations work even on regular block removals. So, if there's only one yellow virus with two blocks stacked atop it and three regular red blocks next to it, a yellow-red block placed properly will still net you a combination. And remember, if you managed to eliminate three, four, or more lines (viruses or otherwise) in one turn, more and more blocks will rain on your opponent's board!