Since it began in March, Yemen’s civil war—bolstered by Saudi airstrikes and a crippling air and sea blockade—has brought about a near-total collapse of Yemen’s public health system. Now, on top of endemic diseases like malaria, Yemen’s coastal cities are witnessing a rapidly-escalating outbreak of dengue fever. Thousands of residents in five cities have contracted this contagious disease, chiefly in the Red Sea coast city of al-Hudaydah and the southern port city of Aden, according to the local health officials. The disease broke out in Aden and al-Hudaydah almost simultaneously, before spreading into other southern cities, reaching the southeastern province of Hadhramawt.

Dengue fever is caused by viruses transmitted to humans by mosquitoes. Though the viruses cannot pass directly from human to human, carriers of the disease infect mosquitoes that bite them, which subsequently pass the virus on to others.

The recurrence of dengue fever in these Yemeni coastal cities, however, seems to have a seasonal pattern. According to Director of Medical Doctors Association in al-Hudaydah, Najeeb Molhi, this infectious disease recurs annually in the city, with most cases occurring between November and March each year.

“The cases [of dengue fever] start emerging in November or December of each year and the peak incidence occurs in March,” said Molhi, who also reported that the ongoing fighting and blockade have made the health situation worse.

Due to both the Saudi-led air war and the civil conflict, the health system in Yemen has been virtually paralyzed: fighting on the ground has caused constant and widespread power outages and fuel shortages, while the months-long sea, land, and air blockade—imposed by the Saudis and their allies—has made it impossible for aid agencies to bring in sufficient medical supplies and fuels to this most impoverished Arab country.

“Yemen’s health system is on the verge of breakdown, and it is only thanks to the heroic efforts of the country’s health workers, the resilience of its brave people and the tireless efforts of national and international humanitarian organizations that any semblance of health care is being provided,” Dr. Ahmed Shadoul, WHO’s Representative to Yemen, was quoted as saying in a WHO press release.

More than 158 health facilities have been forced to shut down, according to UNICEF, and several hospitals have had to reduce their operations to specific units. The Health Ministry in the capital, Sanʻa, have said that at least 62 health facilities have been affected by the relentless air and ground warfare.

“Health infrastructure continues to be hit, with attacks reported on hospitals and ambulances, a medical warehouse, an oxygen factory, and a blood transfusion center,” said the WHO in a recent statement.

Now, roughly one third of Yemen’s population (8.6 million) needs urgent medical help and 80 percent (21.1 million) needs some form of humanitarian assistance, while half of the population is food insecure.

Al-Hudaydah (Governorate population: 2,621,000 people)

Dengue fever was first documented in Yemen in 1870. One of the country’s most significant outbreaks occurred in 1954, affecting 98% of the population of al-Hudaydah. This year in al-Hudaydah, where most of its populace lives below the poverty line, at least 2,700 residents have thus far been infected, out of which ten have died, the Director of Health Bureau in the city, Abdurrahman Jarallah, said in an interview.

Jarallah said that “most of the cases have occurred in the young people, particularly females,” indicating that dengue fever has also reached several rural areas and that tens of cases were not registered.

In early 2011, an infectious disease—known locally as Mukarfas in al-Hudaydah—was controversially identified as dengue fever. That March the disease spread to several rural areas of the governorate, claiming the lives of dozens of people, according to the local health bureau.

A recent study, published by BB Bulletin in February, concluded that dengue in al-Hudaydah was most commonly found among teenage and young adult males. Speaking to the YPP, Murad al-Ahdal, one of the four Yemeni authors of the BB Bulletin’s field study, said that patients observed in al-Hudaydah suffer three phases of the disease:

“Symptoms in the first phase start with fever and joint pains and last for seven days, while in the second phase the complications arise as the platelet count drops down to dangerous levels, along with acute blood loss, which can result in death if the patient isn’t given blood and the infected platelets isolated,” said al-Ahdal, who received top marks from Sanʻa University last year for his M.Sc thesis on how to detect and identify dengue viruses in Al-Hudaydah using PCR tests.

“The third phase is the most dangerous phase of infection; the patient would suffer a shock during the first three days along with blood clots in more than one place in the body,” which can result in sudden death.

Al-Ahdal believes that al-Hudaydah city has been “a hotbed of the dengue fever viruses since 1994, with three identified serotypes, which are transmitted by mosquitos of the family Flaviviridae.” His field-based thesis was the first in Yemen and the region to reveal the fourth serotype of flaviviruses.

Aden (population: 800,000 people)

In the southern coastal city of Aden, where the months-long civil conflict has left residents with virtually no food, clean drinking water, fuels, or sufficient medical help, more than 5,000 people have been infected by dengue fever as of last week.

Khadher Laswar, the Director of the Health Bureau in Aden, told the YPP via telephone that 5,042 dengue fever cases have been registered so far. “At least 219 people of various ages have died.”

The director indicated that the count of cases and fatalities in the areas where fighting is fierce is believed to be higher.

“So many people have been infected but couldn’t reach the hospitals, and many died in their homes,” he said.

Like in al-Hudaydah, sewage and garbage piles help mosquitoes, which transmit the dengue fever viruses, to procreate in Aden, but the relentless fighting, Laswar said, has worsened the health situation in the city despite the concerted efforts of a number of aid agencies.

The ICRC’s Yemen office said that four districts in the city—Crater (Sirah), Khor Maksar, al-Tawahi, and al-Maʻala—are “infested with dengue fever.”

Al-Mukalla (population: 531,205 people)

The port city of al-Mukalla in the southeastern Hadhramawt Governorate has so far seen scores of dengue fever cases, which are believed to have come from Aden. Over the last month, dozens of sick and injured people have arrived in al-Mukalla from Aden and neighboring governorates of Abyan and Shabwah.

Riyadh Jariri, the Director of Hadhramawt’s Health Bureau, said that “al-Mukalla is an infested area, as tens of dengue fever cases have emerged in a number of districts of the city.”

“At least 20 cases have so far been registered in the city, including six cases of people from outside al-Mukalla,” Riyadh said. As more and more Yemenis are displaced by fighting—over one million people have left their homes already—it’s likely that the disease will continue to spread.