If you lack scruples or a conscience, there are lots of ways of making money out of other people’s Macs. Proper malware or ransomware is pretty serious stuff which draws a lot of heavyweight attention from users and law-enforcement agencies, a bit like the human smallpox virus used to be. But like fatal viral diseases, nasty malware will generate a strong response, and may not make you much money.

In human infections, the most successful are the ones which seldom kill the host, but spread well – like colds and flu. Many people will keep going as normally as possible when they are most infectious, ensuring that few of us escape exposure.

That is just like adware. In itself it does not do anything too malicious, just drives you crackers. So you will probably try living with it for a bit, when you discover how hard it is to pin down and remove. You may even be finagled into generating additional income to the adware’s providers before you realise that it is unfriendly.

Adware is now becoming much more common, even on OS X systems which are kept up to date and reasonably well protected from malware. I suppose I provide advice to affected users, through MacFormat, every few weeks, whereas I cannot recall the last time that I had to grapple with proper malware.

If you want to read detailed technical information, one excellent source is Patrick Wardle’s blog at Objective-See. Two of the last three articles are analyses of recent OS X adware, one considering how Facebook click-bait schemes can be used to install adware, the other (a guest post) looking inside Pirrit cross-platform adware.

As adware doesn’t do any immediate harm to your Mac, it is easy to take it more lightly than proper malware, and to treat it as a nuisance. However adware can readily, and without warning, become a powerful vector for the transmission of serious malware. Because it can get so deep-rooted, it is also usually hard to remove completely.

We are becoming quite used to our Macs changing in unexpected ways, as the result of more and more silent updates, from Apple, Adobe, and other official services. When things do change, we often do not have a high enough index of suspicion that it might be the result of adware or malware. If you have an anti-virus product installed, you may assume that it will find and fix anything, but time after time users have been caught out when those assumptions have proved false.

There are some excellent tools available: Objective-See offers TaskExplorer and BlockBlock, in particular, which every Mac user should install and use; Malwarebytes offers Anti-Malware for Mac, which can be invaluable for removing adware thoroughly.

Please use them, and maintain your suspicion and caution. Then if you do catch a touch of adware, it will be more like a brief summer cold, not an outbreak of Ebola.