Scientists have discovered the biggest virus ever, called a pandoravirus. National Geographic has the details on this “alien” virus:

Each about one micron—a thousandth of a millimeter—in length, the newfound genus Pandoravirus dwarfs other viruses, which range in size from about 50 nanometers up to 100 nanometers. A genus is a taxonomic ranking between species and family.

In addition to being huge, pandoraviruses have supersize DNA: 2,500 genes as compared with 10 genes in many viruses. 

The most interesting part, perhaps, was that a pandoravirus was possibly discovered as far back as 13 years ago. However, because anything on the 1 micron scale is of the bacterium size, scientists were dismissive.