Strange connections between computers and the number 13

Friday, October 13, 1989: The first computer-virus panic?

The Jerusalem virus wasn't the only Friday the 13th threat. Another virus, Data Crime, prompted widespread warnings in 1989. One particularly disturbing report described the Royal National Institute for the Blind losing program files and work involving Braille translations.

The victims weren't just people losing games or spreadsheets...

In October 1987, researchers at Hebrew University in Jerusalem discovered a particularly nasty computer virus. Dubbed the Jerusalem virus, it targeted IBM-compatible computers running DOS and spread by infecting executable files—those familiar .COM and .EXE programs.

Once an infected program was launched, the virus stayed in the computer's memory, quietly infecting other programs as they were opened. But it had a special trick reserved for Friday the 13th. Beginning in 1988, whenever that unlucky date rolled around, the virus could delete programs as users attempted to run them.

And it wasn't exactly subtle the rest of the time. About 30 minutes after activation, infected computers could slow dramatically, while an odd little section of the screen scrolled upward, leaving a strange black window. Imagine watching your computer gradually lose its mind and having absolutely no idea why.

The virus also had a programming flaw that caused some infected .EXE files to grow larger every time they were opened. Instead of infecting them just once, it could keep adding copies of its own code until the programs became unusable.

The threat became especially notorious on Friday, January 13, 1989, when infections disrupted computers at British businesses, including British Rail. Antivirus researchers were inundated with calls, and the incident made international headlines.

The Jerusalem virus eventually became one of the most recognizable early DOS viruses, sometimes called Friday the 13th or 1813, after the number of bytes it added to certain infected files.

An actual computer virus programmed to ruin your Friday the 13th. Sometimes the horror stories really do write themselves.

One particularly fun historical detail: in January 1989, antivirus researcher Alan Solomon recalled that media crews were so eager to film a real infected-computer victim that researchers had to disguise a visiting corporate technician as one of their own staff to protect his anonymity.

The Jerusalem virus wasn't some modern ransomware operation demanding cryptocurrency. It was a tiny piece of malicious DOS code, spread in the floppy-disk era, with a built-in calendar and a flair for dramatic timing. And sometimes it managed to break programs simply because its own programming was lousy.

Here's six more connections between computers and the infamous number 13... 

1. The Friday the 13th computer virus was real
In 1987, the Jerusalem virus was discovered infecting DOS computers. On Friday the 13th, it could delete infected executable programs when users tried to run them.

On May 13, 1988, the virus caused a major outbreak, and the date became known in antivirus history as Black Friday. It even made international headlines. Sources: Kaspersky · F-Secure

2. The internet has 13 root server identities
One of the internet's most important systems, DNS, uses 13 named root server identities, labeled A through M.

These help computers find the right destinations when you enter website addresses. Fortunately, there aren't literally just 13 physical machines holding the internet together. ICANN reports more than 1,500 actual servers operating under those identities. Thirteen is an engineering legacy, not a superstition. Source: ICANN Root Server System

3. A famous computer timestamp landed on Friday the 13th
Unix computers commonly represent time as seconds elapsed since January 1, 1970. On February 13, 2009, at exactly 23:31:30 UTC, that counter reached: 1,234,567,890

Yes, that beautifully sequential number occurred on a Friday the 13th. No catastrophe followed. The universe apparently missed an excellent opportunity for a practical joke. Source: Unix timestamp reference

4. Apollo 13 had a remarkable collection of thirteens 
This isn't strictly personal-computer history, but it's too good to leave out. Apollo 13 launched on April 11, 1970, at 1:13 p.m. Central Time. Two days later, on April 13, an oxygen tank exploded, jeopardizing the mission.

The crew survived through extraordinary engineering, navigation, and ground-support efforts. It's an eerie coincidence, although not evidence that 13 caused anything. Source: NASA's Apollo 13 history

5. On Linux, error 13 means "Permission denied"
Now that's an appropriate error message for an unlucky number. On Linux, system error 13 commonly represents EACCES, meaning an operation was denied because of insufficient permissions. The number isn't universal across every operating system, but on Linux it's an authentic bit of wonderfully appropriate computer trivia. Source: Linux manual

6. The number 13 hides inside your Enter key
In ASCII, decimal character 13 is Carriage Return, abbreviated CR. It's a control character descended from mechanical typewriters and terminals. Windows commonly represents line endings with a carriage return followed by a line feed: character 13 followed by character 10. So even if you're not superstitious, 13 may be lurking inside your text files. Source: Microsoft ASCII reference