Micky you're wrong, well according to the guy that hosted the last asbestos awarenes course i went on you are and i've been on two, the second was a more intensive course, i have also been on a 3 day asbestos operatives course too.
In theory it CAN take just one fibre but in the real world you are very unlikely to just breathe in one fibre.
Asbestos dust, consisting of millions of tiny airborne fibers, floats in the air and gets into the lungs. The body responds by forming scar tissue when small asbestos fibres are inhaled. Extensive scar tissue formation is what can lead to lung cancer, asbestosis, and malignant mesothelioma.
Once inside the body, asbestos fibres can increase in size due to ferrugination, or the permeation of iron in the fibre. When an asbestos fibre lodges itself in a lung, the body's immune system tries to fight it.
The body has cells called macrophages in the immune system, which exist to absorb, or eat, foreign particles or bacteria in the body. When an asbestos fibre gets into the lung, the macrophages try to absorb, or eat the fiberes. But remember, asbestos fibres are slivers of crystal rock, so the macrophage is unable to destroy or absorb an asbestos fibre. The fibre continues to travel throughout the lung, causing scar tissue. The macrophage attack on the asbestos fibre also leaves iron deposits and causes the fibre to increase in size due to ferrugination so as you can see it really CAN be just one fibre to kill you