"For several days every time it rains we find ourselves flooded, with all the risks that go with that," said Gervais Itoua, who lives in Mikalou, in the city's sixth arrondissement.
"We get into a terrible state because we know that the water will be carrying excrement stored near our houses," he said.
In many districts of the city there is no proper sewage system. Young men, who come from neighbouring Democratic Republic of Congo, are paid to dig large holes in the ground into which they dump the contents of septic tanks from surrounding houses.
There is also no proper drainage system in these districts. The few gutters that exist were made primarily for the evacuation of household waste water.
So when the heavy rains begin in late October - usually lasting until the middle of December - the holes filled with human waste overflow and the stinking contents are swept out across the streets. Every year there are outbreaks of diarrhoea and typhoid fever.
"The flooded zones will experience serious threats to public health if measures are not taken urgently," said a government health official who asked for his name not to be published. "But the government has long known about this situation."
There is little help from the state. Last year the only aid was in the form of mosquito nets, bedding, mattresses and other domestic items.
Elisa Dimi, director of the Centre Hospitalier in the city's Talangai district, said that in recent weeks she had been seeing a lot of people suffering from illnesses linked to dirty water.
"We don't keep specific statistics on them but we can say that most of them come from the zones that are subjected to flooding after rains," she said.
"These people, most of them children, suffer from Schistosomiasis, Leptospirosis or diarrhoeal diseases. We're also worried about diseases such as cholera, which last year took many victims," said Dimi.
Schistosomiasis, also known as bilharzia, is caused by contact with contaminated water. It causes the slow destruction of some organs. Leptospirosis is also caused by contact with contaminated water and its symptoms include fever, headache, and vomiting, and may include jaundice and diarrhoea.
Another problem caused by the Brazzaville rains is silting. Dozens of houses face being completely swallowed up by sand shifted by the floods.
"Not only are we confronted with the threat of disease, but there is also the threat of being buried in the sand," said Auxence Okombi, a civil servant, who lives in one of the threatened areas.
"A few days ago, had it not been for the vigorous intervention of my neighbours, things might have gone really badly for me. They helped me keep the sand at bay and limit the damage," he said. "We fear the worst in the next rains."