Dear Doctor: Some researchers say the main reason for many health issues today, including allergies and asthma, is that our children are kept too clean and have no resistance to anything. What are your thoughts?

Dear Reader: First, let's discuss the benefits of hygiene. The greatest boon to human longevity, at least in developed parts of the world, has been our ability to deal with wastewater. Separating humans from the waste they produce, and treating that waste, has decreased our susceptibility to a host of diseases. In Yemen, for example, the number of cases of cholera currently exceeds 800,000 due to the inability to manage wastewater. In fact, inadequate sanitation, combined with a limited ability to properly disinfect hands, is estimated to cause 4 percent of deaths worldwide.

But yes, there may be a risk to too much good hygiene. Humans live in symbiosis with the bacteria within and upon their bodies, and an inherent balance is necessary for a healthy life. This balance may extend to the bacteria in our environment, meaning that restricting our bodies from normal outdoor organisms may have a downside. The Amish in the United States, for example, live on farms and spend much of their time outside. Coincidentally, their children have much lower rates of allergies, eczema and asthma compared to non-Amish children who don't live on farms.

The evidence extends beyond that correlation. A 2017 study of adult farmers and their spouses compared their childhood exposure to a farm environment against their rates of allergies and asthma. Those who were exposed to farming as children were less likely to have allergies as adults. Further, those whose mothers worked on a farm while pregnant also had a lower rate of asthma and allergies.

Then there was a Swedish study on pacifier cleaning. Researchers found that the parental practice of cleaning a pacifier by putting it in one's own mouth before giving it back to the infant was linked to a lower rate of eczema and asthma in children at age 18 months. That same study also showed that vaginal birth, with its increased infant exposure to bacteria, was linked to a lower rate of allergies at 18 months compared to Caesarean delivery.

Other studies have shown that having an older sibling decreases a child's risk of allergies and eczema, supposedly because older siblings bring more bacteria into the house. Similarly, having a pet in early life (especially if it's a dog) has been linked to a lower rate of allergies and asthma in children.

But bacteria may just be one factor in the allergy equation. Exposure to allergens themselves may desensitize a child to them later in life. That brings us to the fact that children and their parents overall are spending less time outside. At the same time, children in densely populated urban areas are exposed to pollutants at younger ages. Both factors may increase the risk of allergies and asthma.

In summary, it's clear that the rates of asthma and allergies have incrementally increased in the United States over the last 50 years. Although the research is largely correlative, it seems equally clear to me that we can reduce that risk in our children by getting them -- and ourselves -- outside the house as much as possible, exposing them to their surrounding biome (within reason), and pushing for a reduction in pollution.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. Owing to the volume of mail, personal replies cannot be provided.)

Dear Doctor: My wife had a bad cold, and then I unfortunately caught it. Then, just as I was recovering, she got it again. What's going on? I didn't think it was possible to get the same cold twice.

Dear Reader: It's tempting, and even logical, to think that your wife's cold has ricocheted between the two of you. However, viruses are abundant and opportunistic, so who was infected with which bug, and at what point in time, is up for grabs.

It's entirely possible that you became infected with the same virus that caused your wife's cold. But it's equally likely that you brought home another virus of your own. If so, then on her next go-round, she may have actually caught your cold.

If she did indeed pass along her virus to you, then the second cold she caught was most likely a new and different virus. That's because, thanks to the marvels of our immune systems, it's extremely unlikely to catch the same cold twice. Even if you do manage to become re-infected, thanks to the antibodies provided by your immune system, the new cold takes a significantly milder form.

However, cold and flu viruses mutate readily. When that occurs, despite having developed antibodies to fight off the original cold, the altered virus can indeed make you sick again. Whether your family's illnesses were the result of one, two or even three different viruses, there are simple and effective steps you can take to limit their spread. These precautions are worth implementing at home, at work and while out in public.

Viruses can survive for up to three days on surfaces and inanimate objects. When caring for someone who is ill, don't share food, drink, dishes, glasses or cutlery. Wash everything used by the cold sufferer in hot, soapy water as soon as possible.

Be aware that the books, pens, remote controls, telephones, electronic gadgets and doorknobs that the sick person touches can harbor the virus. A spritz with a bleach solution or a disinfectant cleaner is a good idea. Making sure the sick person frequently washes or disinfects their hands is important. Ditto for the caregivers and all others in the house.

Do what you can to build up your own immune system. Eat well, drink plenty of fluids, take part in regular exercise and get enough sleep. Avoid rubbing your eyes, touching your nose or putting your hands to your mouth after touching public spaces where viruses may linger. And don't forget your annual flu shot.

If you do get sick, please don't try to power through. If your employer offers sick leave, take it. Your co-workers will be grateful. If you have no choice but to go to work, don't try to be a hero. Take advantage of the many over-the-counter remedies that can lower fever and reduce the symptoms that make colds so miserable. And follow good cold hygiene by taking every precaution possible: Sneeze and cough into a tissue, wash your hands often, and don't share food, phones or any objects that may harbor germs, so as not to spread the virus.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. Owing to the volume of mail, personal replies cannot be provided.)

Dear Doctor: My husband was diagnosed with avascular necrosis in his right hip. Can you please tell me what this is and how to treat it? He's not a surgical candidate for hip replacement due to his age, and he doesn't want to take prescription pain medication. What can he do?

Dear Reader: These are smart questions to ask. As the name implies, avascular necrosis of the bone is an interruption of the bone's blood supply. This supply consists of a rich, thin network of blood vessels similar to the small roots of a plant. Because blood supply from surrounding tissues is limited, the bone becomes dependent upon these small vessels to survive; if blood flow is interrupted, the bone nourished by a particular vessel will die. This damages the architecture of the bone and causes a loss of calcium, eventually causing the bone in that area to collapse. Each year, 20,000 to 30,000 people in the United States are diagnosed with avascular necrosis.

Such interruption can be caused by blood clots; circulating fat; bubbles of nitrogen (seen in decompression sickness); damage from fractures or dislocations at a joint; radiation therapy; or inflammation of vessels in conditions like lupus.

Drugs can play a role as well. Anti-inflammatory steroids, such as prednisone and methylprednisolone, increase the risk of avascular necrosis, especially for people treated with high-dose steroids for a prolonged period. In fact, steroids are believed to be a factor in 21 to 37 percent of patients with avascular necrosis.

Alcohol consumption is believed to be a risk factor in 31 percent of cases, with heavy use leading to fatty deposits within the vessels. The more one drinks chronically, the greater the risk of developing the disease.

Avascular necrosis is more likely in people with lupus, antiphospholipid syndrome, sickle cell disease, acute lymphoblastic leukemia or Gaucher disease and in people who have had a transplant. Many times, the cause is unknown.

Avascular necrosis of the hip -- the most likely joint to be affected -- is often the most disabling. The pain initially felt while walking can later occur at rest and, when the condition becomes severe, patients cannot walk at all due to the joint's collapse.

That's not to say treatment might not help. Bisphosphonates, such as Fosamax, have shown benefits in some, but not all, studies. (These drugs also have been linked to avascular necrosis of the jaw.) Iloprost, a dilator of blood vessels, can increase blood flow and has been linked to a delay in the need for a hip replacement. And statins may reduce the risk of avascular necrosis in people taking steroids.

As for pain, core decompression might help. In this procedure, a needle is placed in the bone marrow to encourage bone regeneration. Also, a procedure called osteotomy can alter the bone at the hip so that the area won't be subject to further damage from walking. For most people, however, a total hip replacement is the ultimate solution, resolving symptoms in a majority of patients.

For your husband, I'd recommend drugs to slow the progression of the necrosis, as well as a second opinion from a physician who can help further assess the risks versus benefits of a hip replacement. Age alone generally isn't a deal-breaker. If he truly isn't a candidate for joint replacement, however, his doctor can assess other potential therapies.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. Owing to the volume of mail, personal replies cannot be provided.)