Home > Articles

  • Print
  • + Share This
From the book

Crowding and virulence

Earlier thinking held that, given time, all diseases would adapt, to become no worse than measles and mumps. Virulent diseases were newcomers, not yet adapted to a state of biological détente with their human hosts. This viewpoint sees man and his infections in a perpetual cold war, with casualties due only to occasional misunderstandings. This wishful thinking has obvious marketing appeal and still frequently appears in books and articles that popularize biology.

This scenario ignores the ugly side of both evolution and human history. The inhabitants of our history books did not merely suffer from childhood diseases while their mothers read them stories about rabbits and mice dressed in human clothes. Until our own privileged age, most people died of infectious disease, much of which small rodents spread. The purpose of evolution is not to make life better for humans, nor even to produce a balanced ecosystem. Indeed, the very idea that evolution has some underlying moral purpose is basically religious. Evolution is simply a mechanism by which different living things compete using various genetic strategies. Those that propagate their own kind more effectively increase in numbers, and the less efficient go extinct. Mother Nature has no maternal instincts.

No absolute reason exists for why a disease should not remain virulent, nor why it should not get more virulent. Some do. Indeed, the same disease may fluctuate in virulence as conditions change. The critical issue is which factors promote decreased virulence and which promote increased virulence. The two main factors are overcrowding and transmission mode. Consider again two variants of the same disease, one mild and the other virulent. If humans are closely crowded, the virulent version has the advantage: There is no need for the patient to linger for several days to pass on the germs. As long as plenty of new victims are available nearby, the best strategy is for the disease to grow as fast as possible inside the original victims, generating more germs to infect more people. The slower, milder version of the disease will be left behind. Diseases tend to grow in virulence when their hosts are plentiful and crowded closely together. Conversely, diseases evolve with lesser virulence when their hosts are few and far between.

A highly virulent epidemic may wipe out a substantial portion of the human population. This decreases crowding, which, in turn, selects for a decrease in virulence. Ultimately, you might think, a balance will be struck and both the population density of the host and the virulence of the infectious agent will settle down to a gentlemanly compromise. This is the microbiological version of the famous "balance of nature" myth. But instead of reaching a state of stable equilibrium, periods of population growth generally alternate with devastating epidemics. Chinese records illustrate this effect. Between 37 A.D. and 1718 A.D., 234 outbreaks were severe enough to count as plagues—that's one every seven years. Although not every epidemic covered all of China, the frequency is impressive.

Bubonic plague provides a nice example of a disease whose virulence oscillated. Beginning in the mid-1300s, repeated epidemics of bubonic plague swept across Europe until the 1600s (later in some places). When plague first reached a town or city, the first few cases were usually mild and the victims recovered. Once within the crowded confines of a town, the plague became more virulent, often switching to its pneumonic form, which is spread through the air by coughing. Anyone who caught pneumonic plague could be dead within a day. From the germ's viewpoint, this is no problem, provided humans coughed germs over and infected another victim within this time. In a crowded medieval city, this was normally the case. Toward the end of an outbreak, most of the population either was dead or had recovered and become immune. Hence, the plague became milder again as the number of available victims became fewer and farther between. The mild forms then spread to the next city, and the cycle repeated. After a couple generations, the population recovered to where it could provide a sufficient supply of fresh victims, and the plague might revisit the original city.

Note the time scale. Microorganisms evolve so fast that they can change their minds—or, rather, their genes—during the course of an epidemic lasting less than a year. As illustrated by the Black Death in a single city, mutants with increased virulence may appear and spread in only a few weeks, and the reverse occurs toward the end of the outbreak. Thus, the virulence of a disease such as plague neither decreases nor increases; it oscillates. A major problem for the historian is that if a disease can change significantly in a year, how did it behave a hundred years ago? A thousand? Ten thousand?

Today the human population is exploding. In many Third World countries, this is exacerbated by poor hygiene. Consequently, we can expect diseases that are efficiently transmitted from person to person to become more virulent. In addition, more people need more food. The tendency is to plant larger areas with the same crop, to improve efficiency. However, such crowding makes crops more susceptible to epidemics, just as with humans. The best-known crop disaster was the Irish potato famine, which resulted from over-reliance on a single crop. When a virulent strain of blight fungus wiped out the potatoes, the Irish had little left to eat. Infectious disease then followed in the footsteps of malnutrition. Starvation itself killed relatively few—most victims died of cholera, dysentery, or typhus fever. Thus, crop failures and malnutrition amplify the effects of infectious disease.

Vectors and virulence

Virulence may increase when a vector carries a disease. If a germ hitches a ride from one victim to another via mosquito, it matters little that the first victim is too sick to move. Indeed, this may even work to the germ's advantage. Mosquitoes will be able to land and suck blood without the victim swatting them. Diseases that are carried from person to person by some other agency have little motivation to evolve mildness toward humans. Rather, they must avoid disabling their carriers. What happens to the human victims is less important. Malaria, sleeping sickness, typhus fever, yellow fever, and many other diseases are spread by insects, ticks, or lice. These diseases are dangerous and show few signs of getting milder. Indeed, the more virulent form of malaria, Plasmodium falciparum, spreads throughout the tropics and subtropics from its original focus in Africa.

The best way to control these diseases is to kill the vectors, thus interrupting transmission. Spraying insecticides such as DDT greatly reduced the incidence of malaria in many areas. Sadly, malaria is making a comeback in many parts of the Third World, due partly to insecticide-resistant mosquitoes and partly to complacency and political disintegration. Irrigation projects such as dams, reservoirs, and irrigation canals often work well in temperate climates. However, in tropical regions, they may backfire. They create large bodies of stationary water that are ideal breeding grounds for the mosquitoes that carry malaria, yellow fever, and other diseases. The slowly moving water of canals also provides a suitable habitat for water snails that carry the parasitic worms causing schistosomiasis (bilharzia). An example was the spread of schistosomiasis during the Senegal River Basin development in West Africa.

Waterborne diseases use the water itself as a vector. Such diseases can also increase in virulence. The disease relies on contaminated water instead of an insect to carry it from person to person. But the principle is much the same: The disease does not rely on human victims for dispersal. Contaminated water supply normally spreads dysentery, cholera, and many other infections that cause diarrhea. Rivers can carry germs in untreated sewage downstream and infect towns and villages hundreds of miles away.

  • + Share This
  • 🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020