Isnt not picking up the phone first a part of the sickness?
When a virus invades a cell, it does it stealthily and starts using the cell's own equipment to make more viruses. It doesnt give the cell a chance to "call" for help from the immune system because the cell doesnt even know the virus is there. Just like when we decide to drink, we are in a period of insanity but dont know it, so we wont call.