Dear Doctor: I read that hospitals in the United States might run out of yellow fever vaccine soon. I'm a natural worrier, but I can't help wondering if yellow fever really is an issue in this country.

Dear Reader: It has been more than a century since the last major outbreak of yellow fever in the U.S. But with several recent new outbreaks throughout the world, including one in Brazil, the supply of yellow fever vaccine is running low at a difficult time.

To understand why the resurgence of yellow fever has health professionals concerned, let's take a closer look at the disease.

Yellow fever is a flulike illness spread by certain species of mosquitoes that are infected with the virus. It affects both monkeys and humans, and is carried between the species by the infected insects.

Not everyone who contracts the virus develops symptoms, which can range from mild to severe. In less-serious cases, patients experience fever, chills, headache, muscle aches, nausea and vomiting. Jaundice, which causes the skin and whites of the eyes to appear yellow and can also be a symptom, is how the disease got its name.

In the small proportion of people who go on to develop a severe form of yellow fever -- about 15 percent of those who are infected -- the disease can cause uncontrolled bleeding, and result in heart, kidney and liver problems. According to the World Health Organization, half of the severe cases of yellow fever are fatal.

The disease, which is believed to have originated in Africa, became an international problem in the 1600s, when shipping began to go global. The infected mosquitoes were transported throughout the world, and yellow fever epidemics began to occur.

During the Spanish-American War, five times as many American soldiers died of yellow fever as did in battle. This led to the formation of a commission that proved what had been suspected since the mid-1800s -- that the disease is spread by mosquitoes. Subsequent sanitation programs, followed by the development of a pair of vaccines in the 1930s, led to the near-eradication of the disease in many areas. The last outbreak in the U.S. occurred in New Orleans in 1905.

Now, because of an increase in disease among monkeys, flagging vaccination programs in some countries, and the mosquito-friendly environment caused by a warming planet, infected mosquitoes are on the move again. This has caused a spike in demand for the vaccine. Prior to 2000, about 5 million doses per year were adequate. By 2007, demand rose to 34 million doses of the vaccine.

American health officials, who say the outbreak in Brazil is uncomfortably close to home, are working to expand the supply of vaccine. Sanofi Pasteur, the main vaccine producer, announced plans to open a new manufacturing facility in the U.S. next year.

In the meantime, federal health agencies are working to make available an alternate vaccine that is imported from France. Clinic sites with this vaccine are listed here: wwwnc.cdc.gov/travel/yellow-fever-vaccination-clinics/search.

(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 granddaughter's college offers "morning-after" pills via vending machine. Using these pills without a doctor's prescription and oversight can't possibly be safe, can it?

Dear Reader: Between 2006 and 2010, one out of every nine women of reproductive age used some form of emergency contraception -- often when other forms of contraceptives had failed. Although the subject raises both religious and ethical questions for some people, the reality is that so many women have already been safely treated with emergency contraception by doctors that some forms of the drugs are now available over-the-counter.

Oral emergency contraception works by delaying a woman's ovulation, thus lowering the likelihood that sperm will fertilize an egg. Pregnancy rates among 20-something couples is 30 percent if intercourse occurs one to two days prior to ovulation. If intercourse occurs at any random point in a woman's cycle, the overall chances of getting pregnant are only 4 to 6 percent. Studies of Plan B (levonorgestrel), the medication provided via vending machines at some campuses, show an overall chance of pregnancy of 2.6 percent among women who've taken the drug. So in reality, the drug is only about 50 percent effective in most cases.

Note that, if a woman is obese or overweight, the drug is even less effective. Obese women (those with a body mass index of 30 or higher) were four times more likely to become pregnant after taking Plan B than those with a healthy body mass index. Overweight women (BMI of 25 to 29.9) were two times more likely to get pregnant after taking Plan B than normal-weight women.

Plan B has other limitations as well. For maximum effectiveness, it must be used within 72 hours after intercourse, and it doesn't work as well among women taking some seizure medications or St. John's wort.

That said, the physical side effects of Plan B are relatively minor; they include nausea, vomiting, headache and breast tenderness. Among women who do become pregnant after taking Plan B, there has been no evidence of altered growth or development of the fetus. There is also a low likelihood of birth defects. However, Plan B's biggest downside is that it just doesn't work very well. Plan B may give a false sense of security that a pregnancy won't occur, so women should understand that they have other choices for emergency contraception.

The prescribed medication ulipristal acetate (Ella) has an efficacy rate of about 66.7 percent and can delay ovulation by five days. Further, the drug has been used in Europe for many years and has been FDA-approved in the United States since 2010. The side effects are similar to those of Plan B.

The most effective form of emergency contraception is the insertion of a copper intrauterine device (IUD) by a doctor. This is 95 percent effective at preventing pregnancy, and in a study of 1,013 women who had this device placed in the first five days after intercourse, the pregnancy rate was 2 in 1000.

Of course, many doctors don't provide the copper IUD, and one could argue that a woman may not be able to see her doctor in a timely enough fashion to get an IUD or other form of emergency contraception. Some women may even be embarrassed to see a doctor about emergency contraception.

So, although Plan B may not be as effective as other forms of emergency contraception, it is still moderately effective and quite safe. Those who are obese or taking medications that interact with Plan B should look for other options.

(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: What kind of eye protection should be worn during a solar eclipse? I live in a part of Nebraska where the upcoming eclipse on Aug. 21 will be total.

Dear Reader: The path of totality for this eclipse, which arcs its way from Oregon to South Carolina, will cut a diagonal right through your state. As Californians living 900 miles south of the closest point of totality, we envy you.

We put our astronomy hats on and, with an assist from NASA, learned that the last total solar eclipse in North America occurred on July 7, 1972. (The wait for the next one won't be quite so long -- it's coming on April 8, 2024, and will be visible from Texas to Maine.)

This year, if you have clear skies and live in the Lower 48, you'll get to experience at least a partial eclipse. And that means that for the two to three hours the eclipse lasts, you must take precautions to protect your eyes and eyesight.

A solar eclipse occurs when the orbits of the moon and the sun align in a way that the moon blocks any part of the sun. In your area, you'll get the rare spectacle known as totality, which is when the moon completely blocks the sun's face. During up to 2 minutes 40 seconds of sudden nighttime -- the light will be about as bright as a full moon -- the solar corona, which is the flaming, roiling crown of the sun's atmosphere, will be revealed.

During totality and only then, NASA scientists say it's safe to look directly at the eclipse. (NASA has a great interactive map on its website that pinpoints the arc of totality.) However, before and after totality, you risk damage to your eyes and to your eyesight if you look directly at the sun.

That's due to something called solar retinopathy, which happens when sunlight floods the retina, located at the back of the eyeball. This causes the rods and cones, the light-sensing cells that make vision possible, to release a flood of communication chemicals which can damage the retina. It's often painless, so you don't even realize damage is being done. And remember -- even at 99 percent of totality, the sun's rays can cause retinal damage.

Here, courtesy of NASA and (deep breath) the American Astronomical Society, the American Academy of Ophthalmology, the American Academy of Optometry and the National Science Foundation, are eclipse-viewing guidelines:

-- The only safe way to look at the partially eclipsed sun is through special solar filters like "eclipse glasses," or through handheld solar viewers.

-- Homemade solar filters and ordinary sunglasses, no matter how dark, are NOT safe for looking at the sun.

-- Never use a camera, telescope, binoculars or other optical device, whether filtered or unfiltered, to look at the un-eclipsed or partially eclipsed sun. They will concentrate the solar rays, which will enter your eye and cause serious injury.

-- Full instructions, including the only four certified manufacturers of eclipse glasses, are available at eclipse2017.nasa.gov/safety. We strongly recommend you read (and share) this information.

(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.)