Typhoid mary

[Michiko]

Mary [Mallon], the first known healthy carrier of [typhoid fever], who spread it by working as a cook (typhoid fever is spread through water and food). When health officials tracked her down, she refused to believe she was a carrier, because a. since she was the first known case, shed never before heard of healthy carriers and b. she did not understand a lot about typhoid fever. She was forcibly [quarantined], released after two years on the condition that she wouldnt seek work as a cook again and quarantined again for life after another outbreak was tracked to her working as a cook under a pseudonym. It seems she had taken the job because other household jobs (like [doing laundry]) she got didnt pay as well and she still didnt believe she could spread the disease.
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.
Someone who, analogous to 2, doesnt 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.