Virus
First of all you have to learn that what is a virus . A computer virus is a program which can be load in your computer , blog or server . In that condition your blog runs against your wishes .Your blog or your computer is controlled by the virus code. Viruses can also replicate themselves .
What a virus can do
A virus can do everything which is bad for your blog .
It is and isn't possible for someone to infect your blog with a virus. It is possible if they know how to edit your blog, using your username and password, so they could just imbed a virus to the webpage, but it isn't possible unless they can directly edit your blog.
The Melissa virus is a mass-mailing macro virus. It can effect email that is sent and infect the computer when the email is opened.
useful effect viruses
That entirely depends on the design of the virus.
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Lysogenic virus.
the effect is jail time to the person that set the virus
i think it is not virus...it not effect the pc..
The effect of influenza virus was severe to the extent where average life span in US was depressed by a period of 10 years.
anything expect for wallpaper
They create genetic variations.
Effect