Eugene's observations

Computer viruses used to be something that spotty youths created to impress their peers.  When the screen went red and silly messages or photos appeared you knew you had a virus. 

Then criminals realised there was money to be made and they created infections that would allow the computer to become theirs to command - often for distributed denyal of service attacks used to blackmail large companies.  Now such people have realised that there is more money to be made by not alerting the owner that they have succumbed so they are putting effort into hiding not only the cause of the virus to make it hard to clean up, but also the fact that one is present.

Toda'y virus is often so subtle that you may not notice it.  Most of the time it is inert but just occasionally it will wake up to send a copy of your address book back to base, or use subtle means to trap your password entry then let you continue without any visible effect.

This way the computer will probably stay infected for longer and its control will remain saleable or leasable for longer to generate greater income for the bad people.  So how can you know that your PC (or Apple Mac, Android phone, or IOS tablet) has been compromised ?

• Run routine whole computer virus scans as a matter of course
• Know what level of CPU activity is normal for you
• Monitor unexplained network activity
• Pick up on e-mails purporting to be sent by you or correspondents that have falsified the address
• Detect an unusually high amount of spam mail being received 
• Notice when your password is not recognised even when your know you have it right
• Look for the last login date and correlate it with your own activity
• Any unexpected display/results when you know there has been no other change