UniFoxOs
Where have you been? 100% Hold Baggage Screening was progressively introduced after the Lockerbie bombing way back in the 80's! every checked in bag is screened by some of the most sophisticated equipment available which can detect more than you'd believe possible.
The next attack is likely to be biological, just think a 'carrier' gets onboard an aircraft as a normal passenger and spreads whatever they have to their 100 or more fellow passengers. When that aircraft gets to its destination everyone gets off, looking OK, and disperse far and wide. Within a day or so they all become ill and pass whatever they are carrying onto neighbours, work mates etc...
Remember SARS? not a deliberate attempt to spread a virus but look what it did to air travel when it surfaced.
Now where did I put that surgical mask can't fly without it!