Book Notes: Why We Get Sick

Back in December I read the book “Why We Get Sick” (1992) by Randolph Nesse and George Williams. While some of the information was outdated due to its age, overall I loved the book as it took a more wholistic, evolutionary approach to explaining sickness.

Given the global pandemic of 2019-nCoV (novel coronavirus) underway and the timely nature of my read, here are my brief notes I took from the book.

Why We Get Sick

Two kinds of explanations for disease:

  1. Proximate explanations — Answer “what” and “how” questions about structure and mechanism. Address how the body works and why some people get a disease and other’s don’t. A proximate explanation describes a trait — its anatomy, physiology, and biochemistry, as well as its development from the genetic instructions provided by DNA.
  2. Evolutionary explanations — Answer “why” questions about origins and functions. Show why humans, in general, are susceptible to some diseases and not to others. (Or why some parts of the body are so prone to failure.) An evolutionary explanation is about why the DNA encodes for one kind of structure and not some other.

Defenses. Mechanisms our body and immune systems designed specifically to combat an issue. A protective response to a problem. Coughing is a defense. The distinction between defenses and defects is important — defects are not preprogrammed responses, they are results of a problem. Skin turning blue from lack of oxygen is a defect.

Causes of disease:

  • Infection. External agents such as bacteria and viruses.
  • Novel environments. Environments our evolved bodies aren’t used to handling. A mismatch between our design and our environment.
  • Genes. Some of our genes are perpetuated despite the fact the cause disease. In the environments we evolved in, they didn’t harm us enough not to be selected out. DNA can also be mutated and create new bad genes.
  • Design compromises. There are costs associated with every major structural change preserved by natural selection.
  • Evolutionary legacies. Evolution is incremental and can’t make major changes quickly. Many of the design choices are not optimal and carry on anyway.

Signs and symptoms of infectious diseases

Symptoms of colds and other sicknesses and diseases can be unpleasant. But most of them are useful. It is an adaptation shaped by natural selection specifically to fight infection.

Fever is an adaptation to raise body temperature enough to assist with fighting infection. Body temperature is carefully regulated even during fever; the thermostat is just set a bit higher. Children who take Acetaminophen take about a day longer to recover from chicken pox. There are costs of a fever, of course. Otherwise the body would just always stay at 103F at all times. It depletes nutrient reserves 20% faster and causes temporary male sterility. Still higher fevers can cause delirium and lasting tissue damage. And because regulatory precision is limited, fever will sometimes rise too much and at other times not enough.

Iron withholding. Bacteria lack the ability to get iron easily. Iron is a crucial and scarce resource for bacteria, and their hosts have evolved a wide variety of mechanisms to keep them from getting it. The same chemical that helps induce fever greatly decreases availability of iron in the blood. In the midst of flu, such iron-rich foods as ham and eggs suddenly become disgusting; we prefer tea and toast. Low iron levels can help fight disease.


How does a host guard against infection? (Expanding on the above chart.)

  • Hygiene. Avoid exposure to pathogens.
  • Skin. Erect barriers to keep them out of the body and act quickly to defend and repair any breaches.
  • Pain and Malaise. Generalized aches and pains are also adaptive. They encourage inactivity — which favors the effectiveness of immune defenses, repair of damaged tissues, and other host adaptations. Medication that merely makes a sick person feel less sick will interfere with these benefits.
  • Expulsion and Pain. Flag any cells that lack proof of identity and expel them from their entry portal. The body must have openings for breathing, intake of nutrients, expulsion of wastes, and for reproduction. Each of these openings offers pathogens an invasion route, and each is endowed with special defense mechanisms. The defenses at each body opening can be quickly increased if danger threatens. Coughing, spitting, vomiting, nausea, diarrhea.
  • Mechanisms to attack invaders. If all the above defense lines are breached, it can poke holes in the pathogens, poison them, starve them, or do whatever is necessary to kill them. If it can’t kill them, it can also wall them off so they can’t spread. Macrophages constantly wander the body searching for foreign invaders. If they find one, they transfer it to a helper T cell, which then finds and stimulates whichever white blood cells can make an antibody that binds to the foreign protein. This impairs them and labels them for attack by specialized larger cells.
  • Damage and repair. If all the above still doesn’t work and they’ve caused damage, it can repair it or compensate for it in some way.

In order to choose appropriate treatment, we need to know if the cough, or other symptom, benefits the patient or the pathogen. Instead of just relieving symptoms and trying, perhaps ineffectively, to kill the pathogen, we can analyze its strategies, try to oppose each of them, and try to assist the host in its efforts to overcome the pathogen and repair the damage.

Evolutionary arms race

The relationships between hosts and parasites are so competitive, wasteful, and ruthlessly destructive that arms-race terminology offers an entirely appropriate framework for describing them.

Evolution consists entirely of trial-and-error tinkering. The process is slow and unguided — in some ways misguided — by there is no limit to the precision and complexity of adaptation that the Darwinian process can generate.

Bacteria can evolve as much in a day as we can in a 1000 years. We cannot evolve fast enough to escape from microorganisms. Instead, an individual must counter a pathogen’s evolutionary changes by altering the ratios of its various kinds of antibody-producing cells. Fortunately, the number and diversity of these chemical weapons factories are enormous and at least partly compensate for our pathogens’ great evolutionary advantage.

An unsanitary water supply is only one example of what Ewald calls cultural vectors. The history of medicine shows repeatedly that the best place to acquire a fatal disease is not a brothel or a crowded sweatshop but a hospital. In hospitals, large numbers of patients may be admitted with infectious diseases normally transmitted by personal contact.

Natural toxins

Natural toxins are those that are found commonly in our evolutionary environment.

Our best defenses against these toxins are the same as infectious diseases — hygiene, expulsion, avoidance. Many swallowed toxins can be denatured by stomach acid and digestive enzymes. In the liver for example, specialized enzymes will alter some toxic molecules to render them harmless, and bind to others which are then excreted in the bile back into the intestines. For instance, our protection against cyanide depends on an enzyme called rhodanase, which adds a sulfur atom to cyanide.

Many plants make substances that interfere with the nervous system: opioids in poppies, caffeine in coffee beans, cocaine in the coca leaf. Why do the plants contain these toxins? A few coffee beans might give us a pleasant buzz, but imagine the effect of the same does on a mouse. Potatoes contain diazepam (Valium) but in amounts too small to even cause relaxation. Other plants have toxins that cause cancer or genetic damage, sun sensitivity, liver damage — you name it. The plant-herbivore arms race has created weapons and defenses of enormous power and diversity.

If our livers are overloaded with too much toxins, they cannot process them all and excess toxins circulate throughout the body, doing damage wherever they can. Some exposure to toxins can stimulate increased enzyme production in prep for the next challenge. So perhaps with toxins, as with sun exposure, our bodies can adapt to chronic threats but not to occasional ones.

Grazers and browsers limit their consumption of certain plants to avoid overloading any specific toxin. This dietary diversification also helps to provide adequate supplies of vitamins and other trace nutrients. We minimize the damage caused by dietary toxins by this instinctive diversification, as well as with out own special array of detoxification enzymes.

These enzymes are not as potent as those in goats or deers, but are more potent than those of a dog or cat. We could be seriously poisoned if we ate a deer’s diet of leaves and acorns.

Our enzyme systems can apparently cope with low concentrations of tannin, and many of us like its taste in tea and red wine. Small amounts of tannin may even be helpful by stimulating production of the digestive enzyme trypsin.

Human diets expanded after fire was domesticated. Because heat detoxifies many of the most potent plant poisons, cooking makes it possible for us to eat foods that would otherwise poison us.

A new variety of disease-resistant potato was recently introduced that did not need pesticide protection, but it had to be withdrawn from the market when it was found to make people ill. Sure enough, the symptoms were caused by the same natural toxins the Andean farmers had spent centuries breeding out. An evolutionary view suggests that new breeds of disease-resistant plants should be treated as cautiously as artificial pesticides are.

Novel toxins

Novel toxins are a special problem not because artificial pesticides such as DDT are intrinsically more harmful than natural ones, but because they are so different than what we evolved to cope with.

Furthermore, we have no natural inclination to avoid some novel toxins. Evolution equipped us with the ability to smell or taste common natural toxins and the motivation to avoid such smells and tastes. In psychological jargon, the natural toxins tend to be aversive stimuli. But we have no such machinery to protect us from many artificial toxins, like DDT, that are odorless and tasteless.

Tests on rats are of limited reliability as models for human capabilities, and there are many political difficulties that can frustrate public action on environmental hazards.

There is no such thing as a diet without toxins. The diets of all our ancestors, like those of today, were compromises between costs and benefits.


Embryonic and fetal tissues may be harmed by lower concentrations of toxins than adult tissues are.

Morning sickness is often the first reliable sign of pregnancy. This nausea and its associated lethargy and food aversions are common. For some women they mean many weeks of misery, while others aren’t bothered much.

Nausea and food aversions during pregnancy evolved to impose dietary restrictions on the mother and thereby minimize fetal exposure to toxins. A healthy, well-nourished woman can often afford to eat less. The food she is inclined to eat is usually bland and without the strong odors and flavors provided by toxic compounds. A lamb chop that smells fine to a man may smell putrid and repulsive to his pregnant wife.

Women who have no pregnancy nausea are more likely to miscarry or to bear children with birth defects.

Pregnant women should be extremely wary of all drugs, both therapeutic and recreational. Do not succumb to the urgings of others to eat what you are inclined to avoid.

Certain kinds of clay, as mentioned in the discussion about acorns, tightly bind soluble organic molecules, including many toxins. In other words, they may relieve symptoms in the best way possible — by removing the harmful cause.

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