It is not just technical tricks that help viruses to spread further and faster, some users are doing their part too, warns technology analyst Bill Thompson.
Like any other profession, programmers do not appreciate criticism.
Herd immunity gives strength in depth
So my comments last week about software 'engineers' who do not really deserve the title as they produce shoddy, bug-ridden programs that have to be continually patched and repaired prompted a number of e-mails complaining that I had unfairly criticised a dedicated and hard-working group of people.
As it is important for any writer or critic to be balanced and fair, I think it is only reasonable to admit that programmers are not the only cause of the many problems which face today's net users, from viruses to phishing scams to worms and Trojan horses.
Users are just as bad, and may actually be worse because, while most programmers try to do their best, it seems a lot of users are just wilfully stupid.
This was brought home to me very clearly this week as I was sitting on the floor of a particularly overcrowded evening train from London Kings Cross to Cambridge.
Opposite me were two men of about my age, both talking loudly about computers, the internet and technology.
How could I fail to take an interest?
After a fascinating discussion about the best programs to use to download unlicensed MP3s over the net, and how to copy the resulting files to a minidisc player or other portable device, the conversation turned to viruses and other nasty programs.
They were both well aware of the Sasser worm, and the threat it posed to all net users, and one of them was a bit worried that his laptop might have become infected.
"Don't worry'" said his mate "you can go online and get the free 30 day trial of the anti-virus program and clean up your PC."
[Those] who think it is okay to freeload on other people's attempts to control viruses are the Typhoid Marys of our online age
This concerned me slightly, since anti-virus software does not cost that much, and it does actually do the job.
But then came the killer blow, as he leaned forward and confided: "And whenever something big blows up there's always a free tool you can get to clean your computer, so you don't really need to worry."
I almost choked on my beer.
It seemed quite clear that these two men, who would probably think nothing about spending £30 on a meal out, were either too mean or too stupid to realise that there are lots of viruses and worms out there that never make the headlines.
Relying on these free single-virus tools would only guarantee that their PCs were loaded with adware, spyware, Trojans, worms and probably software that lets a hacker use a remote system to attack other sites, turning it into a zombie.
Sadly, we then reached Letchworth and they moved to sit down somewhere else in the carriage, so I did not catch the rest of their conversation.
If their approach to viruses is typical, then we are in big trouble.
I did not get infected by Sasser, and I have only had one virus on my laptop in the three years I have owned it, because I am careful.
It is not just humans that need immune systems
But I have been seriously inconvenienced by other people's viruses, sending me masses of infected e-mails, clogging up network connections and forcing companies to put all sorts of blocking measures in place that often stop me sending e-mail to people I need to contact.
And it is the fault of people like them, reckless about the ways their infected PCs can affect other people.
The real argument here is about herd immunity, the idea that if enough members of a group are immune to a particular disease, like measles or mumps, then even if some catch it there will never be an epidemic, and the spread will always be limited.
As a parent of two children, I have followed the arguments about the MMR vaccine and the dangers it poses with extreme interest.
I also know that the whooping cough vaccine, the meningitis vaccine and all other inoculations and vaccinations carry a small risk.
That risk, however, is far less than the risk posed by the disease itself, and each of us can, by taking that small risk for our own child, reduce the overall incidence of the diseases involved to the point where they can be effectively disregarded as a potential threat to our children.
So it is with our computers.
The Sasser author was arrested in his home town
It would be nice if programmers wrote better code, if software development techniques advanced beyond the primitive approaches to provability, bug checking and security auditing we see today, and if companies were forced to accept liability for their poor code instead of forcing us to accept licenses which effectively absolve them of all responsibility.
It would be nice if we could be sure that every patch issued by Microsoft would work as advertised and not cause problems of its own. But the world is not like that, and at the moment the patches are the best hope we have.
Bugs exists, in software and in the real world.
Programs can be crashed by malicious hackers who want to break into secure systems. And children will catch measles and meningitis.
Not doing everything you can to protect your own computer by installing a firewall, buying anti-virus software and keeping it up to date and installing the patches issued by vendors when they come out rather than when the worm that exploits them is everywhere on the net, is as anti-social as refusing to tell your sexual partners that you have got Chlamydia.
And the guys on the Cambridge train who think it is okay to freeload on other people's attempts to control viruses are the Typhoid Marys of our online age - thoughtless, careless of others, and almost certainly infectious.
Bill Thompson is a regular commentator on the BBC World Service programme Go Digital.