Potentially the biggest threat to the security of your business is the actions of your employees. Every business needs a defined security policy and that policy should be made available to your users in both written form and, wherever possible, through end user training.
However, no matter how security aware your users are, there is a high probability that simply by browsing the internet and visting the wrong (or even correct) websites might introduce a threat into your network through spyware, viruses and other malware. Not least access to the internet for some individuals may result in reduced productivity if given free access to go whereever and do whatever they like.
Also your users may receive "spam" emails that can have many negative effects including introductions of viruses and trojans to the network as well as exposing them to "Phishing" style attempts to engineer personal information from them.
Your traditional first line of defence, the firewall, may be of no benefit in addressing these threats as HTTP (web) and SMTP (email) type traffic will normally be allowed through the firewall. Effective defence against these threats requires the application of some or all of the following:
Modern firewalls, especially Unified Threat Management (UTM) firewalls, may provide increased protection and control over these elements.
You also have the option to deploy a stand alone solution to mitigate these threats.