Diphtheria, a deadly infectious disease once thought to have been largely eradicated, has now joined cholera as a public-health scourge threatening war-torn Yemen, where a blockade by Saudi Arabia has impeded emergency aid.

Officials at the World Health Organization said Friday that at least 22 people in Yemen had died of diphtheria and nearly 200 had been sickened since it was detected three months ago.

The disease, which the medical charity Doctors Without Borders said had not been seen in Yemen for 25 years, has now spread to 13 of Yemen’s 22 governorates.

Officials warned that young children were especially vulnerable to the disease, which spreads through the air and could escalate quickly into an epidemic in Yemen if health workers there lack the antitoxins and vaccine to control it. Currently they have little of either.