Not every pest makes itself felt.
There are many different types of malware. If the goal of the attack is, for example, identity theft (your user IDs + passwords), remote control of your computer for further attacks (your computer becomes part of a Distributed Denial of Service (DDoS) attack or a botnet) or the mass sending of SPAM, then the attacker will of course do everything possible to make sure that you do not notice the attack or infection – and that works.
There is no such thing as 100% security against infections. However, if you keep your virus software up-to-date and also keep all other software packages up to date, are careful when clicking on links and opening e-mails and downloading software only from known, trustworthy sources, you are already a long way ahead.
The German Federal Office for Information Security (Bundesamt für Sicherheit in der Informationstechnik, BSI) provides further tips at https://www.bsi-fuer-buerger.de/BSIFB/DE/Service/Checklisten/Massnahmen_gegen_Internetangriffe.html?nn=7904670.
Tips for dealing with e-mails: https://blogs.tu-braunschweig.de/it/?page_id=3165&preview=true