A computer virus by definition is self replicating software with malicious intent.
A virus beyond itself can be in a couple of forms. A worm or a Trojan. A virus typically is contained in an executable file. You could have a virus now, but not be infected. You can think of it like a human virus. As a scientist you have a virus stored within a vial or container for testing purposes. This does not mean you are infected. With that understanding, the virus living in your system could be contained within an executable file, innocuous until triggered by executing the file containing it.
Worms on the other hand are a bit different. A worm doesn't require your interaction to spread it's joy. By design, a worm is intended to take advantage of a system's services. This type of infection can and will spread over a network either local or wide area. It can spread via email or other system services.
A Trojan, aptly named based on the Trojan Horse event in which the Greeks entered Troy by way of trickery, is either a standalone virus or worm delivered to the end user in a deceitful manner. You feel you're getting a 'gift' or acting on something legitimate. Trojans are commonly delivered to their victims either by email or a webpage. Sometimes it's not specifically the webpage, but rather one of the ads within the page that entices the victim to act on it.
Another term widely used is 'Malware'. Malware encapsulates all types of infection and adds to the array, spyware, adware and ransomware.
Spyware is embedded in the host system and tracks users, or rather it spies on user's activity. It could be as simple as snooping on your browsing activities, or it could be as bad as logging your personal information such as bank accounts and credit cards. These sorts of infections are intended to be stealthy and keep the end user unaware of their presence.
Adware is advertisement based infection. Typically once the user is infected with this type they will be bombarded with popups and advertisements. Usually for services that are not needed. These advertisements are not limited to browsers and website surfing. Often times these 'ads' will start appearing after system startup and continue with browsing activity. A common front of these kinds of infection is to look official and dupe the user into thinking they are infected. You could say in this respect they are the most honest types of infection. However their rogue promises of cleaning the false infections they report, is for no other purpose then to relieve you of money.
Ransomware is one of if not the worst type of infection there is. This gem, upon being activated encrypts or makes your data inaccessible until you pay a ransom. Usually the ransom can only be paid via Bitcoin or some other payment method that is untraceable and without a way to cancel. Even after paying said ransom. Which is typically a large sum that grows larger the longer you wait. Users rarely get their data back. We (DSI) have some success getting the data back, but it requires data recovery tier 1 or 2.
Infections vary in intensity and danger. Some infections are nothing more than a nuisance classified as a 'Potentially Unwanted Program' (PUP). These are mild forms of adware usually. Some infections can bring a corporate network to its knees, steal a person's life savings, or destroy digital memories cherished and irreplaceable.
Virus infection is not limited by operating system platforms. While commonly most infections are Windows platform based, Linux based as well as Mac based can and do get infected. These later infections have been increasing more and more over recent years as these platforms become more mainstreamed especially in the portable device / cell phone market.
99.9 percent of the time the purpose of infection is greed based. Somewhere along the line of time happiness was redefined and encapsulated with dollar symbols. While money is helpful, it certainly should never be a goal. Especially at the expensive of preying on the weak or tricking the uninformed.
So what can you do? A lot actually. Common sense cannot be encouraged enough. The old adage, "if it seems too good to be true, it probably is" should be considered like gospel. Although some of the trickery methods are hard to spot. You may get an email that looks to be from your bank or a social media website you have an account with. The email may look 100% legitimate. The unsuspecting user will be easily fooled and thus start the infection process thinking they are doing the right thing when in fact it was a false claim.
If you think, or if you know that you are infected, best practice suggests that you immediately shut down your system and bring it in to have it professionally diagnosed and remedied. Delay in this action simply causes more damage and problems. If your not sure you can use this free tool to run a quick check. Though it is a quick simple scan it may give you some answers. If your still not sure, come see us.
Proper software goes a long way in this battle of good vs evil. DSI recommends to our customers 360 Safe products, either Total Security Essentials or Total Security. At present and over the past 2 years we have used it, 360 Safe is freeware. Meaning there is no cost for the software. Coupled with the premium version of Malwarebytes (which you can purchase at any of our stores including our online store). These two software packages are light weight so they run well on lower cost systems. They are extremely effective when mixed in with common sense at combating one of the leading problems of computer based technology today.
Most folks are very disappointed when they have to come see us because their better known paid software packages such as Norton, McAfee, AVG, etc has not done the job. Though some of this relates to the 'common sense' factor we talked about earlier, the fact is (in my opinion) these packages are better at being marketed than actually fulfilling their roles.