Story highlights Eli Jacks is too young to be vaccinated; his sister has leukemia

Measles cases that have spread to Arizona have put the children at risk

(CNN) Anna Jacks checks her baby's forehead over and over again. Is he hot? Does he have a rash? Is his nose still runny?

Her son has been sick before, but this time it's different: Last week Eli was at a Phoenix Children's Hospital clinic with a woman who had the measles, which spreads easily from person to person. Now he's showing signs of the virus, such as runny nose and cough and fatigue.

At 10 months old, Eli is too young to get vaccinated and would be especially vulnerable to serious complications of measles, such as deafness and brain damage or even death. But his parents have an even bigger worry. If Eli does have the measles, he could give it to his 3-year-old sister, Maggie, who has leukemia.

JUST WATCHED Measles: Why not vaccinating your kids can be deadly Replay More Videos ... MUST WATCH Measles: Why not vaccinating your kids can be deadly 01:29

So far Maggie is feeling fine, but her parents know that with her immune system wiped out by chemotherapy she's even more vulnerable than her brother to complications.

"My biggest fear is that I'll lose my child, or that she'll become deaf," Anna Jacks said. "My family has been through enough with cancer. I don't want her to go through anything else."

Read More