The Rules of Contagion

of people affected by something – whether a disease or another
event – might change over time.
Ross suggested that there are two main types of happening. The
first type affects people independently: if it happens to you, it
generally won’t increase or decrease the chances of it happening to
someone else afterwards. According to Ross, this could include
things like non-infectious diseases, accidents or divorce.[49] For
example, suppose there is a new condition that can randomly affect
anyone, but at first nobody in the population has it. If each person has
a certain chance of becoming affected every year – and remains
affected from that point onwards – we’d expect to see a rising pattern
over time.

Growth of an independent happening over time. Example shows

what would happen if everyone had a 5 per cent or 10 per cent

chance of being affected per year

The curve gradually flattens off, though, because the size of the
unaffected group shrinks over time. Each year, a proportion of people
who were previously unaffected get the condition, but because there
are fewer and fewer of such people over time, the overall total doesn’t
grow so much later on. If the chance of being affected each year is
lower, the curve will grow more slowly initially, but still eventually