If I was an Islamic terrorist I'm not sure I would be seeking to use biological or nuclear weapons. A thumbnail consideration of the majority Muslim nations brings to my attention that all I can think of have high numbers of people concentrated in congested cities with large numbers housed in slums with poor sanitation and few medical resources.

Combine that with the religious demands of the Haj, the journey to Mecca at least once in a life, where Muslims from around the world travel from and then return to their communities and you have a formula for the very communities they ostensibly fight for to be the hardest hit by any contagious biological weapon.

On the nuclear side I would point out that the Muslim holy cites, Mecca, Medina and others, aren't exactly hidden. I don't advocate any such counter attacks and I don't know how the Uma, the Muslim people, would feel about it but I doubt facing in the direction of a smoking radioactive pit and praying would be as attractive as the situation they enjoy now. The unleashing of a deadly contagion that wiped out large numbers of their own population wouldn't make them popular either.

I don't know how terrorists think but IMO they aren't stupid, unaware or unable to think through such things. They may lust to bringing down the western world but not if the destruction primarily lands on their own people.

On the off chance they don't find the above logic convincing I would try to avoid probable targets and large concentrations of people. A goodly supply of appropriate masks, gloves and disinfectants wouldn't hurt either.