The recall of more than 200 million eggs over possible salmonella contamination may have consumers nervously checking symptoms or worried about every stomach twitch.

Almost two dozen people have already been sickened after the eggs were sold in nine states, the Centers for Disease Control and Prevention reported.

READ MORE | 200 million eggs recalled over salmonella fears; how to check your cartons

Each year, salmonella causes about 1.2 million illnesses, 23,000 hospitalizations and 450 deaths in the U.S., the agency estimates. Most people who are infected will feel some misery, though the severity of symptoms can vary, experts say.

“You’re going to feel pretty crappy,” Dr. Joseph Galati, a digestive tract expert and medical director of the Houston Methodist Center for Liver Disease & Transplantation in Texas, told TODAY.

“It’s going to be a spectrum of illness related to the amount of bacteria that you got into your system… from mild, gastrointestinal upset to more of a full-blown (illness).”

WHAT ARE THE SYMPTOMS OF SALMONELLA?

The basic three are:

  • diarrhea
  • abdominal cramps
  • fever

The problems usually appear 12 to 72 hours after eating a contaminated food, the CDC notes.

Doctors will be able to tell if it's a salmonella infection by examining a sample of the patient’s stool or blood. They may also be interested in his or her food history.

HOW IS SALMONELLA INFECTION TREATED?

It is a self-limited infection, meaning it generally resolves on its own, Galati said. The illness usually lasts four to seven days and most people recover without treatment. But some may have diarrhea so severe that they need to be hospitalized. In about 5 percent of cases, the infection may spread from their intestines to the bloodstream, requiring antibiotics to stop organs from being infected, the CDC notes.

Some populations are more vulnerable to infection than others.

“With any sort of illness like this, the extremes of age are going to be more difficult. The young and the old are going to get hit the hardest,” Galati said. “(It also depends on): What’s the status of your immune system?”

WHICH FOODS MAY CONTAIN SALMONELLA?

Surprisingly many, according to the CDC. They include:

  • beef
  • chicken
  • eggs
  • fruits and vegetables
  • pork
  • sprouts
  • processed foods, including nut butters, frozen pot pies, chicken nuggets and stuffed chicken entrees.

Contaminated foods usually look and smell normal.

HOW IS SALMONELLA SPREAD?

The vast majority of the illness — 94 percent — is transmitted by food, the CDC estimates. It’s spread by the fecal-oral route, meaning people usually become infected by eating foods contaminated with feces from an infected animal. But the bug can also get into food through cross-contamination, environmental contamination or by the unwashed hands of food workers.

Salmonella can also be spread by direct animal contact, and rarely, from person to person.

A worrisome new study found chlorine, commonly used to decontaminate fresh produce, can make foodborne pathogens like salmonella undetectable. When the bugs encountered "environmental stresses" such as chlorine, they responded by entering a dormant state and couldn't be seen by standard lab culture techniques. This may help explain outbreaks of salmonella and listeria in produce in recent years, according to the study published Tuesday in mBio.

HOW CAN SALMONELLA BE PREVENTED?

The CDC offers these tips:

  • Do not eat raw or undercooked eggs, poultry or meat.
  • Avoid raw or unpasteurized milk or other dairy products.
  • Wash produce thoroughly.
  • Wash hands, cutting boards, counters, knives and other utensils thoroughly after touching uncooked foods.
  • Keep uncooked meats separate from produce, cooked foods and ready-to-eat foods.
  • Everyone, but particularly children, should wash their hands after handling reptiles, birds — including baby chicks and ducklings — or their environment.