When a giant wall of ocean water rose up from the sea and slammed into the Philippines on Friday, it caused immediate devastation, obliterating entire cities and crippling hospitals and transport in the central part of the archipelago. Thousands are reported dead or displaced.

Far-flung coastal communities accustomed to the Pacific Ocean’s mighty onslaughts were flattened by Typhoon Haiyan, one of the most intense storms currently on record, with sustained winds ripping through their streets at around 320 kilometers per hour and gusts reaching 370 kph. Pres. Benigno Aquino III declared a State of National Calamity on Monday, underscoring the damage to a country that was also hit by a magnitude 7.1 earthquake last month.  As international agencies scramble to bring shelter and supplies to the typhoon-impacted areas, the country is facing formidable public health hazards.

The storm ravaged some of the country’s modern hospitals, including the regional hospital in the city of Tacloban, some 580 kilometers southeast of Manila, where essential medical supplies washed out to sea. A top priority for aid groups is distributing basic medicines to manage chronic conditions like diabetes. Workers are also fighting to get shelter, safe water and sanitation facilities into place even as the nation faces the added threat of a tropical storm later this week. Remote areas of the country, such as the city of Guiuan at the southernmost tip of Samar Island, have remained inaccessible.

Streets rendered impassable by storm debris have prevented the removal of bloated bodies, fueling a psychological toll. The corpses themselves do not pose much of a danger—the risk of bacteria-spread disease from them is low—but the stench can impact the community and aid workers alike, hampering the response.  “I know the Philippines government is trying to maximize law and order in these areas and civil society is trying to take the lead on [removing the bodies],” says Tom Price, a spokesman for Catholic Relief Services, an aid and development group that has worked in the Philippines for decades.

Aid groups now are talking about the country’s vulnerability to waterborne diseases. Poor sanitation and crowded environments can cause explosive outbreaks of diarrhea-causing infections in the aftermath of man-made or natural disasters, sometimes with fatal results. Outbreaks of cholera, for example, can occur if there are endemic levels of the disease in the area, which is the case in the Philippines. (Cholera can also be imported, which happened after the 2010 earthquake in Haiti.) The intestinal infection caused by toxigenic Vibrio cholerae can rapidly spread when human waste mixes with water. It causes severe dehydration or death from acute watery diarrhea, sometimes accompanied by vomiting.

With that in mind, the World Health Organization has emphasized that securing water purification tablets remains essential because crowded living conditions propel infectious diseases, which can exacerbate other health problems. As civil society groups and international forces move in to help, they face huge challenges of no power, security or even water. “It’s like traveling to outer space,” Price says. “The aid workers need to bring everything in, even for themselves, which is slowing everything down.”

All communication has been cut in hundreds of towns and villages stretched over thousands of kilometers in the typhoon's path, which makes it difficult to prepare aid shipments. “Right now we’re operating in a relative black hole of information. We know from the very little we can see that the situation is terrible. But it’s what we don’t see that’s the most worrying,” Natasha Reyes, Doctors Without Borders' emergency coordinator, in the Philippines said in a statement. “To be honest, no one knows what the situation is like in these more rural and remote places, and it’s going to be some time before we have a full picture.”

“At this point we are just trying to get shelter into the impacted area. People are still out in the open,” Price says. His group is preparing to hand out 32,000 mainframe tents and 32,000 hygiene kits—typically with one kit per family—containing water purification tablets, basic hygiene items, soap, toothpaste and items like plastic buckets.

Once shelter and basic sanitation is in place there will still be long-lasting threats. The long tail of the typhoon’s destruction is also stoking anxiety about the future of agriculture in some areas of the Philippines. Because the storm damaged giant swathes of the country’s fisheries and rice paddies, the destruction could put the nation at further risk for future food security,  the Food and Agriculture Organization of the United Nations warned. The typhoon hit at the beginning of the nation’s prime rice-planting season, and the organization estimates that hundreds of thousands of hectares of rice have been destroyed.