LETTERS
the terms should have been reversed.) original  strain. Therefore, knowl-
Contagion on
3) Trojan horses. A Trojan horse is a edge of biological infections can be
the Internet
program concealing harmful code that used to predict and anticipate highly
usually makes a computer or network virulent computer infections.
To the Editor: Computer viruses
available to unauthorized users in an Although the computer user has
are designed to be pests, proliferating
appealing or unsuspicious package. A some recourse against computer
in uncontrolled ways and causing
virus, worm, or Trojan horse can be viruses, the costs may be high. As
severe damage to electronic data.
latent (then also called a logic bomb) with biological viruses, good hygienic
These malignant programs, which
and become active only after a certain practice is helpful. Just as they should
amplify between files and computers,
period. wash hands frequently, avoid exposure
are strikingly similar in virulence,
Each class of computer malware to people with colds, or use condoms
modes of spread, and evolutionary
has hundreds of variants, and many to protect against infectious diseases,
pathways over time to the microbes
variants have several slightly modified computer users should mistrust (and
that cause infectious diseases. Both
versions, paralleling microbial diver- thus not open) files received through
biological viruses and these virtual
sity. Worms, such as the infamous unexpected channels or with unknown
viruses are transmitted from host to
 ILOVEYOU worm in 2000, may extensions or subject lines, request
host. Computer viruses are a human
employ a universal message of gratifi- confirmation from the sender before
invention; however, their development
cation to entice users. Their wide dis- opening attachments, and regularly
follows a well-recognized biological
semination parallels the spread of back up hard disks to reduce the risk
route. Relatively harmless ancestors
socially transmitted diseases (e.g., of losing data. The consequences of
gradually or step-by-step evolve into
influenza) that have the potential to such actions in terms of time, disk
 pathogens; the host develops adap-
infect everyone susceptible. In con- space, and efficiency illustrate a bio-
tive defense mechanisms, which in
trast, computer viruses (spread by logical truth: immunity has cost.
turn select for new virus  variants;
sharing data on infected diskettes) par- Effective antiviral barriers are impedi-
eventually, equilibrium is reached
allel sexually transmitted diseases, ments to communication. Moreover,
between infection and host defenses.
whose spread is related to specific virus protection programs are only as
Comparing  virtual microbes with
behavioral practices. Viruses or good as the last virus recognized, pro-
their biological counterparts can help
worms that are spread undetected but viding only partial protection at best.
us control both.
are activated at a later date (as was the Computer users have not always taken
The term  computer virus is
case with the Michelangelo virus, dis- inconvenient precautions, even in
loosely used to describe computer
covered in 1991 and still around) view of serious consequences.
 malware, an umbrella term that
resemble latent microbes, such as HIV. ILOVEYOU was a worm that used the
includes the following categories. 1)
Denial-of-service attacks, which block same mechanism of spread as Melissa,
Viruses. A computer virus is a pro-
access to a server by an onslaught of which had been released a year earlier.
gram that implants a version of itself
messages, are the equivalent of toxins, Yet, ILOVEYOU turned out to be
in any program it can modify. The
since neither can reproduce in their even more destructive than Melissa.
modified program, once run, attempts
host and are only harmful above a crit- Biological immune disorders in
to modify other programs directly
ical concentration. Spam (unwanted which host defenses turn against the
associated with it. Computer viruses
but harmless e-mail), the curse of host and actually cause damage are
spread by sharing data on infected
computer users with slow modems known as autoimmune diseases. Com-
disks or diskettes. Unlike their biolog-
through expensive telephone connec- puter autoimmune disorders parallel
ical counterparts (which are fast and
tions, resembles bacterial commen- their biological counterparts.
very infectious), computer viruses
sals that can injure the host only under Recently, a warning (defense mecha-
spread slowly and infrequently
specific conditions. nism used by computer users) turned
between computers. 2) Worms. A
Biological viruses can mutate rap- out to be a not-so-harmless hoax. The
worm is a self-contained program that
idly, create novel pathogenic and hoax warning stated that certain files
replicates itself and sends copies to
transmission routes, and develop anti- were infected by a computer virus.
any connected computer, with little or
genic variation to evade host immu- Heeding the warning, unsuspecting
no user interaction. Unlike biological
nity. In the computer world, worms computer users removed the affected
worms (which spread slowly), com-
exhibit similar behavior. Once a worm utility files from their computers
puter worms spread rapidly and with-
has been transmitted successfully, operating systems. The harm mediated
out much user interaction between
variants quickly emerge. These vari- by this  host defense was relatively
computers of a network, including the
ants cause damage in similar ways but small in this particular case, resem-
Internet. (In view of the contagious-
evade detection and impediments bling the discomfort of allergies, in
ness of biological worms and viruses,
installed to provide  immunity to the which immune responses to benign
Emerging Infectious Diseases " Vol. 8, No. 3, March 2002 335
LETTERS
agents cause limited damage. How- However, because of the immensity of trates how pathogens emerge and
ever, more malignant forms of  auto- the threat, the Internet community spread in our increasingly small
mutilating hoaxes are likely to responded extremely rapidly. Within world. The speed of virtual pathogen
emerge that could be as devastating to hours after its release, alerts to system evolution makes it possible to follow
computers as some autoimmune dis- administrators on how to block the the process of mutation and selection
eases are to humans. worm had effectively slowed its in real-time. With countless inter-
The electronic monoculture that spread. Early surveillance and barrier linked computers, the risk for virtual
improves communication also development averted disaster. As in contagion is so great that urgent steps
increases the risk for contagion. Pre- contained epidemics of hemorrhagic are needed to avoid catastrophe. How
dominant use of a single operating fevers, the immense threat of high many pandemics will it take before we
system has improved communication contagion and lethality prompts effec- accept the risks and costs of computer
and sharing of electronic data but has tive measures to rapidly recognize immunity? Similarly, to protect
also facilitated ready amplification of outbreaks and prevent pandemics. against emerging pathogens, we must
virulent programs. As with biological The types of measures to be used use all tools available, including vir-
infection, transmission of computer against computer contagion can be tual pandemics. A task force to collect
infection depends on susceptibility of learned from biology. Immune effec- data on the epidemiology of virtual
the population. Virus producers saw tors of plants and animals protect infections as a model for infectious dis-
an opportunity in the popular prefer- against a broad range of pathogens; eases might be an important first step.
ence worldwide for PCs with however, in nature this system
Trudy M. Wassenaar* and
Microsoft Windows operating sys- evolved over millions of years. Engi-
Martin J. Blaser!
tems. The enormous popularity of neering protective computer systems
these systems, along with their long- with similar efficacy within a few
*Molecular Microbiology and Genom-
recognized inadequate protection years is a great challenge. Current pro-
ics Consultants, Zotzenheim, Ger-
against misuse, made computer users tection programs mainly resemble
many; Virtual Museum of Bacteria,
susceptible. Virtual viruses able to innate immunity, but programs that
www.bacteriamuseum.org; ! New York
infect multiple operating systems are learn from exposure (thus resembling
University School of Medicine and
rare (as are biological viruses with adaptive immunity) are under devel-
New York Harbor VA Medical Center,
broad host specificity), and even when opment. Vaccination with relatively
New York, USA
infected, computers that run on differ- harmless microbes primes the immune
ent operating systems (e.g., Mac, system. Biological hosts also naturally
Unix) or other-than-Outlook e-mail carry protective microflora that com-
programs usually are dead-end hosts pete with pathogens. Could we pro-
Emergence and
for PC viruses. duce  virtual vaccines that are
Rapid Spread of
Pathogens do not reinvent the beneficial to the computers carrying
wheel. Virulence genes are constantly them (e.g., by blocking preferred sites
Tetracycline-
 stolen and reused. Thus, new com- of entrance for viruses or repairing
Resistant Vibrio
binations of virulence genes can result viral damage automatically) and let
cholerae Strains,
in new pathogenic strategies, and such these  good microbes circulate on the
combinations frequently accumulate Internet just as malignant viruses do?
Madagascar
in pathogenicity islands. Reuse and Crude versions of such vaccines have
combination of effective (and infec- already been developed. Recently, a
To the Editor: The Indian Ocean
tive) strategies are also common in worm by the name Fixing the Holes
was free of cholera for decades, until
computer malware. A recent example was discovered that utilized known
January 1998, when an outbreak was
demonstrates the value of just the right security holes to spread to other hosts.
detected in Comoros Islands (1). On
amount of virulence. A highly danger- Using  good microbes would have its
March 23, 1999, the Malagasy Epide-
ous worm called Nimda (Admin in costs: occupation of Internet capacity
miological Surveillance System
reverse) was released exactly 1 week and consequent slowdown of data
reported the first case of cholera in
after the September 11, 2001, terrorist transmission and presence of mali-
Mahajanga, a harbor on the northwest
attack in the United States. Nimda cious worms disguised as beneficial
coast (2). In May 1999, the Malagasy
combined the most powerful strategies ones to elude detection.
sanitary authorities set up sanitary bar-
of Code Red and SirCam and spread Knowledge of infectious diseases
ricades at the borders of the two prov-
more rapidly than any previous worm. may help control computer contagion.
inces Mahajanga and Antananarivo
Clicking on the subject line of an Conversely, study of computer mal-
 affected by the epidemic. Oral dox-
infected e-mail (to delete it, for ware may help curb infectious disease
ycycline was systematically given to
instance) itself activated the worm. emergence. Internet contagion illus-
336 Emerging Infectious Diseases " Vol. 8, No. 3, March 2002