Vaccines revolutionized modern medicine. They all but eliminated scourges such as smallpox, measles, polio and yellow fever from the developed world (though measles has experienced a resurgence). All told, they’ve saved millions of lives.

There are still plenty of dangerous diseases that have no vaccine — but scientists are working on that.

Close to 300 vaccines are now being developed to prevent and even help treat a variety of diseases and conditions according to the Pharmaceutical Research and Manufacturers of America (PhRMA). (Vaccines that treat diseases are known as therapeutic vaccines.)

While the bulk of them are for infectious diseases and cancer, there are a handful in the works for allergies and neurological disorders. More than 200 are already in clinical trials, according to PhRMA.

Here are some notable ones to watch for:

Zika. Zika is transmitted by infected mosquitos and suspected to cause the serious birth defect known as microcephaly. Athletes and would-be spectators who planned to go to the summer Olympics in Brazil, the epicenter of the virus, have to weigh the risk of contracting the virus. Currently the only way to protect yourself is to avoid being bitten.

With countries including El Salvador, Colombia and Ecuador warning women not to get pregnant, it’s little wonder that coming up with a vaccine for this virus is so crucial.

Though President Barack Obama recently asked Congress for $1.8 billion in emergency money to fund an expedited Zika vaccine, it'll be a while before we have one. Marie-Paule Kieny, PhD, assistant director-general for health systems and innovation for the World Health Organization, told CNN that while several companies are working on Zika vaccines, they are at least 18 months away from large-scale trials.

HIV. Since the first HIV vaccine trial opened in 1987, researchers have studied more than 50 different preventive vaccines, but still none is ready and approved, according to the National Institute of Allergy and Infectious Disease (NIAID). However, a therapeutic vaccine currently being tested may delay the disease’s progression, according to PhRMA.

Influenza. Researchers are working on a so-called “universal” flu shot that would provide protection against all strains of the flu and last for more than a year, according to NIAID. The CDC recently reported the flu vaccine saved more than 40,000 lives over nine years.

Related: Flu is on the Rise and Hitting Young and Middle-Aged Hard

Lung cancer. Trials are underway in Cuba for a non small-cell lung cancer treatment vaccine, according to Roswell Park Cancer Institute, which signed an agreement with the U.S. Food and Drug Administration to bring the vaccine to the United States for clinical trials.

Shingles. There’s already a shingles vaccine, but a new version would fix a big limitation in the existing one: The new vaccine is safer for older adults, who may be most prone to the disease, according to a study published in the New England Journal of Medicine. It may, however, cause more pain at the injection site and more frequent muscle pain and headache, according to Harvard Medical School.

Addiction. Immunization against narcotic addiction works differently than immunization against diseases, according to the National Institute on Drug Abuse (NIDA). “Anti-drug vaccines … attach to drug molecules, forming a compound molecule that is too big to cross the blood-brain barrier easily. By slowing drugs' entry into the brain, the vaccines reduce or prevent the euphoria that promotes addiction,” writes Nora D. Volkow, MD, NIDA director.

Alzheimer’s disease. This vaccine had a phase one clinical trial where it was "safe and well tolerated by participants," according to the Alzheimer’s Association. The vaccine, which would stop this debilitating disease from advancing, is still being researched.

Needle-less vaccines

They may be a long way off, but edible vaccines will make it cheaper and easier to protect people against diseases, NIAID says. Just imagine swapping a needle for a fruit-flavored gummy inoculation, particularly when you’re taking your kids for their shots.

Also in development: skin patch vaccines, which are being tested to combat a range of illnesses including travelers’ diarrhea, tetanus and anthrax, according to NIAID.

Related: Nine Things You Should Know About the HPV Vaccine

What vaccines does your family need?

The CDC offers a printer-friendly lifetime vaccination schedule for all ages to make sure everyone in the family remains up to date.

Kathleen Heins is a Greenville, S.C.-based writer specializing in health, well-being and organizing whose work has appeared in Better Homes and Gardens, Ladies’ Home Journal, Woman’s Day, USA Weekend and Reader’s Digest, among other publications.