The filter media filters out organisms that are bigger than their pore size, like parasites, Giardia, amoebas, cryptosporidia, organic material, etc. Many filters have pore sizes small enough to filter out bacteria. The pore size of portable hand operated filters is not small enough to filter out viruses. In general, viruses do not exist by themselves in water. They are usually found in clusters around organic material.
Most filters do a good job of removing organic material and consequently, most viruses.
Some other water filter manufactures make a filter that is impregnated with an iodine resin. The idea here is that the silver-coated elements kill viruses as they pass through the filter and deter bacterial growth on the filter element. The only problem with this theory is that under normal field conditions, water is pumped through the filter too fast to be sterilized by the iodine resin. Thus some filter manufactures such as MSR have decided against making a silver resin coated filter element because it gives the illusion of protection. The best solution is to pre-treat water with iodine for 20 to 30 minutes and then filter the water.