4 ms·
As far as I was told, it is not the strains are mild or not, there are in total 4 strains, you get immune with each one you get, but each it will be worse.
by andi999 6y ago
As far as I was told, it is not the strains are mild or not, there are in total 4 strains, you get immune with each one you get, but each it will be worse.