'Viral' simply means that the condition is caused by a virus, rather than by bacteria (or, say, from a physical injury).
Bacterial infections are relatively easy to treat, since antibiotics can target a wide number of different bacteria.
Viruses are much harder to deal with because they live within the body's own cells. Further, every virus requires a different antiviral drug to attack it (or, more strictly, to inhibit its development so that the body's own immune system can deal with it more quickly).
Viruses can also mutate quickly, so what works for the dealing with the original virus soon no longer works. A typical example is the flu virus (or, more accurately, set of viruses). While antivirals exist which can help the body to deal with swine flu, or Asian flu, etc, those viruses will inevitably mutate and new antiviral drugs will need to be developed.
With many viral infections there would be little point in researchers spending millions of pounds developing an appropriate antiviral drug because, by the time that they'd developed i,t the virus would have mutated (rendering the new drug useless).
Chris