How the youthful Harlots curse
Blasts the new-born Infants tear
And blights with plagues the Marriage hearse
The meaning of these three lines seems to be this. If men consort with prostitutes (harlots) they are very likely to pick up sexually transmitted diseases. The most common and most terrible of these diseases was syphilis, which was incurable. Then when some of these infected men got married, they would almost surely transmit the disease to their brides, who often were innocent young virgins. And, of course, when the wives became pregnant they would transmit the disease to their infants. Blake imagines the infants being so riddled with the disease that even their tears would be contaminated. Eventually a whole population could become infected, as is the case in modern times with AIDS in some parts of Africa.
According to the eNotes Study Guide on Blake's "London":
By choosing syphilis as the symbol for all that is wrong with England, Blake is able to condemn institutions and emotions that are sacred to most people: love and marriage.
Henrik Ibsen's play Ghosts (1882) is about how a young man's life is ruined by the fact that his father passed on the disease of syphilis to him at conception some twenty-six years earlier.