Probably just assumed to be regular pneumonia or the flu or similar.
Some researches in Italy are now trying to go back to see if a higher number of pneumonia cases between October and December 2019 may have actually been COVID, before we knew what COVID was.
U.S.Italian researchers are looking at whether a higher than usual number of cases of severe pneumonia and flu in Lombardy in the last quarter of 2019 may be a signal that the new coronavirus might have spread beyond China earlier than previously thought.
https://www.reuters.com/article/us-health-coronavirus-italy-timing-idUSKBN21D2IG[https://www.reuters.com/article/us-health-coronavirus-italy-timing/italian-scientists-investigate-possible-earlier-emergence-of-coronavirus-idUSKBN21D2IG]