in the early days, the World Health Organization severely underestimated the potential dangers of the virus and completely downplayed the situation so that they wont upset their chinese overlords bribing them with money.
From my understanding, there are 2 strands of corona virus, one that spreads slower and one that spreads faster. More than likely, in the early days they've seen mostly the 1st strain which was less contagious but then it mutated slightly which let it spread faster.