杏吧原创

Who dose?

If there are more than 200 different viruses capable of causing the common cold in humans, is it possible to catch several colds at once, with each one being caused by a different virus? Does immunity against one virus work against any of the others?

鈥� As far as I am aware there has never been any research into whether we can be infected with more than one common cold virus at the same time, but I don鈥檛 see why it shouldn鈥檛 be possible. Our bodies are often invaded by more than one virus, which is why we can have a cold and a cold sore.

But it鈥檚 unlikely that the same cell could be infected by more than one cold virus, as cells produce a substance called interferon when under attack, which protects them from further invasion until that infection passes. If the different viruses were targeting different cells, however, there appears to be no reason why this couldn鈥檛 happen.

Rhinoviruses, responsible for up to 80 per cent of cases, infect only a few cells, and the infection usually involves only a small portion of the cells of respiratory epithelium, leaving many cells available for infection by other viruses. So it seems it would be possible to catch several colds at once, each one being caused by a different virus targeting different cells. You would probably not be aware of such simultaneous infections, though you may feel worse than if you had only one infection, as more of the respiratory apparatus would be infected.

The body has basically two types of immune reactions to a viral infection: non-specific and specific. Once infected, your body will first use the non-specific system, which fights off the intruder using a general mechanism that is deployed against all invaders. If this is not effective and the virus persists, the specific system is used.

The specific system recognises the virus and produces specific antibodies. Recognition is usually based on identifying the complex molecules of the virus (proteins, glycoproteins or complex polysaccharides), otherwise known as antigens. Once the antigens are detected, the body teaches cells called B and T lymphocytes to fight them. Some of the new B lymphocytes turn into 鈥渕emory鈥� cells which can survive for several decades, so the next time the antigen is detected, the system can respond immediately.

To answer the second question, it is the specific system we are concerned with, and whether immunity would work against other viruses depends how closely related the viruses are. The more closely related, the more likely the immunity would work.

Thanks to Marc Van Ranst, Head, Laboratory of Virology, Catholic University of Leuven, Belgium

Killer chemical

How does chlorine in swimming pools kill harmful organisms, and why is it the chemical of choice?

鈥� Chlorine is not the only member of the halogen group that can be used to disinfect water; iodine and bromine will also do the job, though not fluorine because it is too reactive. Chlorine is often chosen simply because it is cheap, readily available and relatively easy to handle.

Disinfection relies on disrupting a harmful organism鈥檚 metabolism or structure. That can be achieved by oxidation and non-oxidising chemicals which have similar effects, as well as by non-chemical processes such as ultraviolet (including sunlight), X-rays, ultrasound, heat (as in pasteurisation), variations in pH and even storage to allow organisms to die naturally.

Chlorine gas consists of molecules of two chlorine atoms but no oxygen. When added to water, one of the atoms forms a chloride ion. The other reacts with water to form hypochlorous acid, an oxidising agent.

Disinfection comes from the hypochlorous acid reacting with another molecule, most probably in the bacterial cell wall, in an oxidation-reduction reaction. If this happens enough times, the organism鈥檚 repair mechanisms are overwhelmed and it dies. So concentration of disinfectant and the length of time pathogens are exposed to it are important factors.

Chlorine is available in many different chemical forms, such as chlorine gas, sodium hypochlorite powder (often used in home swimming pools), and chlorinated lime or bleaching powder. Some chemicals containing chlorine are not disinfectants because the chlorine in them, usually in the form of chloride, is completely reduced with no further oxidising power. Sodium chloride is such a chemical, which is why water cannot be disinfected using a pinch of salt, and why pathogens can survive in seawater.

Philip Jones

Water Environment Consultants
Woking, Surrey

鈥� Disinfection needs to be carried out under closely controlled pH conditions, ideally between 7 and 7.6. If the pH is too low 鈥� less than 6.8 鈥� there is a tendency for nitrogen compounds, especially urea (a common pool contaminant) to degrade via another route to chloramines. The worst of these is nitrogen trichloride, which irritates the eyes and creates the so-called chlorine smell associated with poorly run or overused swimming pools.

Philip Stainer

Lach Dennis Consultants Haverhill, Suffolk

鈥� Chlorination removes contamination immediately in the pool, whereas ultraviolet and ozone treatment work in the plant room. All these systems also use filters to remove organic matter. The less turbid the water, the lower the dose of chlorine needed to sterilise it. So a constant, low level of chlorine can be maintained in the water circulating between pool and plant room, and it can be altered as pool users and pollution levels vary.

Lois Vickers

Bideford, Devon

With thanks to Roger Cole, Proton Water Services

This week鈥檚 questions

Girl power

I am a geologist with three daughters. Nearly all my contemporary male geologist friends also have a preponderance of daughters. During the time our children were born, we all spent long periods working away from home. I am told that deep-sea divers also tend to father more daughters than sons. Are there any studies to support this or is it just a coincidence?

David Gamble

North Balwyn, Victoria

CFC-free

I read that Britain has a mountain of discarded refrigerators because there is a lack of facilities to destroy the CFCs in the foam insulation. I would have expected the CFC gases in the foam to have long since diffused out and been replaced by air. If not, why not?

Peter Byles

Hinton-in-the-Hedges, Northamptonshire