Answers to last issue's Do You Know?

(Courtesy: Scientific American, 2009 and 2010 issues)

1. Is cholera dangerous ? Why and how does it spread ?
Cholera spreads mainly through faeces-contaminated water.  The faeces get into the food or water supplies through mainly the water. Thus it is a big problem in countries without access to reliable sanitation and clean water. 
The cause of the disease is the bacterium Vibrio cholerae, which releases a toxin that triggers severe diarrhea and rapid dehydration; both effects can quickly prove deadly.  Cholera still infects some three million to five million people worldwide and kills at least 120,000 each year, mainly in India and sub-Saharan Africa. 
Cholera can kill within a matter of hours. The main risk factors are people whose stomachs, for whatever reason, are not making the normal amounts of gastric acid -- if someone has recently had stomach surgery or is taking drugs that inhibit the production of gastric acid. The other risk factors are genetic, which unfortunately there isn't much you can do about. If your blood type is O, you're at higher risk. 
In terms of risk for death, it is higher for people who don't have treatment available. If they don't get treatment in a very short amount of time, they have a very high risk of dying.

2. Is it possible to control one's dreams ?
In the movie Inception, the actors use drugs to trigger specific dreams in people. This is not just Hollywood fantasy but an active area of research today.
What does sleep really mean? After all you are still breathing so not everything is shut off. You may not be surprised that breathing, digestion and other functions go on automatically even during sleep.  However, parts of the brain (the cortex) also does not "go to sleep" through the entire duration when the body is asleep.
The literal definition of a dream is a "narrative experience" (story) that occurs during sleep. REM (rapid eye movement) sleep is generally the time during sleep when most of the cortex is as active as it is when we're awake. During this phase, there are rhythmic bursts of activity in the brain stem.
In fact, on average, there are several areas of the brain that are more active during sleep than they would be during the waking state. Those are parts of the visual cortex, parts of the motor cortex and certain motion-sensing areas deeper in the brain.
That's probably why dreams are so very visual (almost larger than life) and also why they have a lot of motion and action in them relative to our waking experience.
That we can control our own dreams is quite true and really much more so than people seem to know or realize. The details of how to do it are very different depending on whether you're trying to induce lucid dreams, whether you're trying to dream about particular content or whether you're trying to dream a solution to a particular personal or objective problem.
Another really common application has been in trying to influencing nightmares. Some nightmares occur repeatedly after trauma of some kind (after an accident, for example, or a really nasty experience). Many researchers are finding that it is possible to influence the perception wile awake in order to stop the nightmare from repeating, or else to transform the nightmare into something less stressful. For example, to induce the nightmare to become one that can be controlled by the person who is dreaming. Once the sleeper knows that she has mastery over a dream, it does not frighten her any more.

3. Athletes exercise to reduce body fat. How far can we do this, how low can we reduce body fat to ?
Apparently Apolo Ohno, the American athelete who has won several Olympic Gold medals, is almost pure muscle: only 2.8 percent of his body consists of fat.
Nutrition experts suggest that a body-fat content of 15 percent is healthy for men and 22 percent correspondingly for women. For athletes, these percentages tend to be considerably lower. So how low can body fat get ? After all, fat is crucial for normal physiology -- it helps support the skin and keep it lubricated, cushions feet, sheaths neurons, stores vitamins, and is a building block of hormones.
Experts think 2 percent would be a safe lower limit if athletes keep proper diet, consisting of good protein sources and some carbohydrates. But while this is a physiological limit, they do not support this approach for athletes. They warn that achieving this range presents health risks, including increased risk of infection and injury. 

4. Why have we not evolved eyes in the backs of our heads (when that would have been very advanageous to us) ?
It is true that having eyes in the backs of our heads would have helped us detect predators that approach from behind. But evolution is not directed towards developing perfect organs. Just because some feature seems like a good idea, random mutation and selection will not necessarily fashion it.
The first light-sensitive cell most certainly arose through random mutation among the earliest multicellular creatures. This mechanism of detecting light conferred a selective advantage, however minute, to those individuals possessing these cells. The best evidence that this is an advantage is the fact that variations on the theme of visual acuity evolved dozens of times, independently, in various invertebrates. Hence there are at least nine variations of the eye known to occur, including the camera lens version.
Although light-sensitive cells are likely to have appeared in different parts of early forms of life, selection seems to favour those that enable creatures to detect light in the direction they are headed rather than the direction from which they came. The simplest reason for this appears to be that creatures prefer to look where they are going, that is, forward!
This was probably the driving force for the current location of light-sensitive cells. Besides, we do have a certain amount of peripheral vision so that we can see people (or creatures) approaching us from behind out of the "corner of our eyes". 
In addition, with a simple 90-degree pivot of the head, we can see behind us without turning our entire bodies around. There was thus probably no need for eyes at the back of our heads.
Having said all that, when I was small, I always thought my mother (and later, one of my teachers) could very well see what was going on behind her !

5. If carbon dioxide is causing gloabl warming, why don't we split it into carbon and oxygen, both of which are useful to us ?
Yes, this is a great idea ! Only, doing so requires energy. Where do we get this energy from ? If we used hydrocarbon fuels, we would end up with more carbon dioxide (CO2) than we started with.
Consider the proposal as a chemical reaction: CO2 plus energy yields carbon and oxygen. This formula essentially reverses coal combustion (carbon plus oxygen yields CO2 and energy). If energy from coal were applied to drive the decomposition reaction, more CO2 would be released than consumed, because no process is perfectly efficient.
Another option would be to harness a carbon-free energy source (such as concentrated sunlight) to drive a high-temperature thermal reaction that does not merely undo the combustion process but instead uses carbon dioxide as an input to generate useful, energy-rich products.  Scientists are trying out such approaches now.