Size of pathogens

One feature of the range of pathogenic organisms listed in table is the enormous variation in size. Viruses are the smallest infectious organisms, being 20–400 nm in size. At the other end of the scale some parasitic worms, such as the tapeworm, can be up to 7 m (20 ft) in length. This represents a difference in scale of a factor of 109. To put that into some sort of perspective, if a virus were the size of a tennis ball, a fully developed tapeworm would reach from London to Los Angeles. It does not stretch the imagination too far to appreciate that the problems posed to the immune system by these two organisms would require very different solutions.