The virus that ate through Da5id's brain was a string of binary information, shone into his face in the form of a bitmap -- a series of white and black pixels, where white represents zero and black represents one. They put the bitmap onto scrolls and gave the scrolls to avatars who went around the Metaverse looking for victims.

The Clint who tried to infect Hiro in The Black Sun got away, but he left his scroll behind -- he didn't reckon on having his arms lopped off -- and Hiro dumped it into the tunnel system below the floor, the place where the Graveyard Daemons live. Later, Hiro had a Daemon take the scroll back to his workshop. And anything that is in Hiro's house is, by definition, stored inside his own computer. He doesn't have to jack into the global network in order to access it.

It's not easy working with a piece of data that can kill you. But that's okay. In Reality, people work with dangerous substances all the time -- radioactive isotopes and toxic chemicals. You just have to have the right tools: remote manipulator arms, gloves, goggles, leaded glass. And in Flatland, when you need a tool, you just sit down and write it. So Hiro starts by writing a few simple programs that enable him to manipulate the contents of the scroll without ever seeing it.

The scroll, like any other visible thing in the Metaverse, is a piece of software. It contains some code that describes what it looks like, so that your computer will know how to draw it, and some routines that govern the way it rolls and unrolls. And it contains, somewhere inside of itself, a resource, a hunk of data, the digital version of the Snow Crash virus.

Once the virus has been extracted and isolated, it is easy enough for Hiro to write a new program called SnowScan. SnowScan is a piece of medicine. That is, it is code that protects Hiro's system -- both his hardware and, as Lagos would put it, his bioware -- from the digital Snow Crash virus. Once Hiro has installed it in his system, it will constantly scan the information coming in from outside, looking for data that matches the contents of the scroll. If it notices such information, it will block it.

There's other work to do in Flatland. Hiro's good with avatars, so he writes himself an invisible avatar -- just because, in the new and more dangerous Metaverse, it might come in handy. This is easy to do poorly and surprisingly tricky to do well. Almost anyone can write an avatar that doesn't look like anything, but it will lead to a lot of problems when it is used. Some Metaverse real estate -- including The Black Sun -- wants to know how big your avatar is so that it can figure out whether you are colliding with another avatar or some obstacle. If you give it an answer of zero -- you make your avatar infinitely small -- you will either crash that piece of real estate or else make it think that something is very wrong. You will be invisible, but everywhere you go in the Metaverse you will leave a swath of destruction and confusion a mile wide. In other places, invisible avatars are illegal. If your avatar is transparent and reflects no light whatsoever -- the easiest kind to write -- it will be recognized instantly as an illegal avatar and alarms will go off. It has to be written in such a way that other people can't see it, but the real estate software doesn't realize that it's invisible.

There are about a hundred little tricks like this that Hiro wouldn't know about if he hadn't been programming avatars for people like Vitaly Chernobyl for the last couple of years. To write a really good invisible avatar from scratch would take a long time, but he puts one together in several hours by recycling bits and pieces of old projects left behind in his computer. Which is how hackers usually do it.

While he's doing that, he comes across a rather old folder with some transportation software in it. This is left over from the very old days of the Metaverse, before the Monorail existed, when the only way to get around was to walk or to write a piece of ware that simulated a vehicle.

In the early days, when the Metaverse was a featureless black ball, this was a trivial job. Later on, when the Street went up and people started building real estate, it became more complicated. On the Street, you can pass through other people's avatars. But you can't pass through ............