Since protecting your computer is as important as anything it might be nice to know that there is also a PowerShell cmdlet available to manually update your virus and malware definitions:
So for example if you would like your Windows Defender definitions to be updated every time a new PowerShell window is opened the following code could be added to your PowerShell profile:
I added in the -AsJob so the updating will take place in the background and PowerShell is immediately available to use. To add this to your current PowerShell profile you could execute the following code:
Add-Content -Path $Profile -Value "`r`nUpdate-MPSignature -AsJob`r`n"
The next time you launch PowerShell it will automatically update your definitions: