Why am I losing around 1000 hairs a day since mid of December 2017?

It's hard to say if the disease that you had to cause hair loss, I recommend you to see as soon as possible a specialist doctor in the field tricho science.

It's true that you could have an acute telogen effluvium but other diagnoses need to be ruled out. Yes, the range is wide because humans are not the same and hair follicles react differently.  For some people, even serious life threatening illness or infectious causes no shedding whereas for others even a slight minor viral infection can trigger shedding. This is especially true for mononucleosis - (where some are affected for weeks and others for months and months).  Humans react differently to countless numbers of factors and so do hair follicles. The problem with assuming that the shed is entirely due to the mono is what to do "if" things do not improve. If you are worried, get an expert opinion through a comprehensive examination. 

Massive hair loss induced by an illness needs to be addressed by a doctor who specializes in hair. Doctors on this site who do hair restoration usually have the expertise to diagnosis and treat your situation