Pertussis, or whooping cough, which has claimed its 10th death in California, is caused by bacteria that can lead to severe upper respiratory infections. Here's what you need to know.

What are the signs?

Whooping cough's symptoms are hard to distinguish from the common cold - runny nose, congestion, sneezing, red, watery eyes, a mild fever and coughing. The coughs may have a high-pitched "whoop" sound when the sick person is trying to draw in the next breath of air.

But whooping cough might be a misleading name because many people — particularly infants (who have been mainly affected by this outbreak) — don't develop the characteristic whoop.

The cough can last for weeks and children can cough so hard and rapidly that blood vessels can burst and they have difficulty eating, drinking and breathing.

Who's at risk?

Nine of the 10 deaths occurred in babies younger than 8 weeks old, which means they were too young to be vaccinated for this highly contagious bacterial disease, according to the California Department of Public Health. Infants are not supposed to receive the pertussis vaccine until they're at least 2 months old.

What is herd immunity?

A segment of the population (too young, allergic or sensitive to vaccines) have to rely on other people who have been immunized to protect them from spreading the infection.

"We need herd immunity and need the vaccination rates up at a certain level," said Dr. Jason Glanz, a senior scientist and epidemiologist at Kaiser Permanente's Institute for Health Research. "It's generally understood if 90 percent is vaccinated, the vulnerable are protected. The children too young to be immunized are particularly vulnerable."

However, some parents are choosing to not vaccinate their children. In other cases, previously vaccinated children and adults may have lost their immunity because the vaccine has worn off.

Who isn't getting vaccinated?

That brings up two different issues. Those who make a conscious decision not to get the vaccinations for whatever reasons are unimmunized. Then there are people who are under-immunized, who tend to be in the lower socio-economic group and have received some vaccines. They typically want health care but may not have access.

"The more people that are unimmunized, you break down your herd immunity and you're more likely to have outbreaks," said Dr. Robert Frenck, professor of pediatrics in the infectious diseases division at the Cincinnati Children's Hospital Medical Center in Ohio. "The people who are choosing to be unimmunized are getting the benefits of the people who are taking the risks of being vaccinated."

What are other factors for the outbreak?

Peaks in whooping cough cases cycle every two to five years. California saw its last peak in 2005, with 3,182 cases, according to state health officials. That number has nearly doubled this year - California has 5,978 confirmed, probable and suspected cases of the disease.

What should I do?

Get vaccinated if you haven't.Make sure your children are immunized.

Since the whooping cough vaccine does not protect you for life, booster shots are needed. They're recommended for kids around age 11, and for adults around every 10 years.

California epidemiologists estimate 50 percent of the children who have gotten sick were infected by their parents or caregivers.

Anyone coming into contact with newborns need to be vaccinated, to create a "cocooning strategy" where the newborns are protected because the older people around them have been vaccinated and therefore won't pass it on to little babies.

Source: CNN