2. A carrier of any contagious disease who, through ignorance and recalcitrance or callousness, proceeds to take exactly those actions which are likely to cause the highest possible amount of infections in other people.
3. Someone who, analogous to 2, doesn't bother with anti-virus software or firewalls and so quickly gets their computer infected with all manner of computer viruses which then proceed to spread themselves from that computer. Especially if they persist in opening all attachments and not installing anti-virus software even after having been told they should.
2. Mao seems to have been a typhoid Mary, according to accounts by his physician stating he had an STD which caused him little discomfort but which he spread at a rate of one young female follower infected per week.
3. If someone alerts you to a virus mail you sent them, and you do nothing about it, that makes you a typhoid Mary.