The polluted Athi River has formed a long disease corridor as it snakes its way into the Indian Ocean, our two-month investigations have revealed.
The Nation also learnt that the government received several reports linking the river's pollution to a number of diseases that have sprung up along its route in the past 10 years.
Machakos Governor Alfred Mutua said a confidential report linking the explosion of cancer among residents living along the river's route to its pollution was presented to the Cabinet when he worked in the national government.
He said his county was dealing with an explosion of cancer, skin diseases -- mostly rashes -- and a host of waterborne diseases.
Residents and health practitioners in Nairobi, Kiambu, Makueni, Kitui, Taita-Taveta and Kilifi counties who share the river expressed similar sentiments.
During our investigation, visits to several health facilities, firmed up by laboratory test findings, confirmed the prevalence of these conditions.
The poison and bacteria we found in the water, vegetables, fish, crabs and other animals can cause various ailments, including anaemia, kidney and liver damage. They can also damage almost every organ, and even the immune system.
Some can also hamper mental and physical development in children.
In the past decade, the face of disease has changed rapidly in Kenya, with respiratory diseases toppling malaria from the top spot.
Last year alone, there were 21 million incidents of diseases of the respiratory system.
One of the main reasons for this increase is heavy metal poisoning, which has a huge impact on the lungs.
Cholera, pneumonia, diarrhoea and stomach infections also shot up the index scale.
"There hasn't been a month that we have handled fewer than 70 cases of diarrhoea," said Elijah Gachuki, a clinical officer at Mukuru Kwa Rueben Health Centre in Nairobi County.
Data shared by the facility, which is a stone's throw away from the river, indicate that it treats about 84 diarrhoea patients every month.
"We are seeing a lot of waterborne diseases lately. For instance in May, we had a mini cholera outbreak in the settlement," he adds.
Counties at the coast bear the greatest burden of the pollution upstream.
Daniel Kizito Ngumbao has lived next to the Sabaki Bridge all his life. Sabaki is where the Athi River drains into the Indian Ocean.
He has buried his sister and a niece, both of whom died of cancer. For over 10 years, he has wondered how they got throat and mouth cancer.
His house is in the last village along the Athi River, now Sabaki River. Ngumbao's family, like hundreds others along the riverbank, grew up eating food grown using water from the river.
Besides skin infections, diarrhoea and other waterborne diseases, our investigation found that residents of the eastern region are victims of slow, heavy metal poisoning, whose consequences take a while to be felt.
But not everyone we spoke to said they had been harmed by the river.
For instance, Mr Vaati Mbiki, said: "I have never been affected by drinking or using its waters. For the more than 30 years I have used it, I have never fallen sick.
"This water does not make anyone sick. Animals also use it but they do not get sick." Mr Mbiki is a resident of Miondoni Bridge, Wamunyu, Mwala Sub-County, Machakos.
He drinks water directly from the river.
In Katangi, Machakos County, Peter Mulwa, 23, said 2014 was the worst year. "We would get skin diseases, which were manifested by itching, scaly and dry skin. Today, sand harvesters are getting such diseases," he said.
In Kilifi County, residents use water from Athi River for various functions, including drinking and cooking.
Urinary tract infections (UTIs), diarrhoea and pneumonia were the most common diseases affecting thousands of residents in Chakama.
The UTIs affect mainly adults while diarrhoea and pneumonia are common among children.
"The diarrhoea is caused by dirty water. Mothers start weaning their babies very early. They do not practise exclusive breastfeeding, hence exposing their babies to diseases," a source at Chakama dispensary in Kilifi County said.
The residents rely on the Athi-Galana-Sabaki River, the second longest river in Kenya spreading to Kilifi.
"We drink directly from the river; our livestock also drink the same water. We treat the UTI through traditional medicines. But our conditions worsen during rainy seasons," Paul Mbela, a Chakama resident said.
Kilometres away, Kakoneni village residents are grappling with skin diseases, mostly fungal infections, especially among children. Diarrhoea is also common.
At Madunguni dispensary, the incidence of UTI, diarrhoea and malaria is high. Sanitation is also poor. "We use the bush as a toilet," said Ms Mwajuma Swaleh, a resident.
By Paul Wafula, Winnie Atieno, Charles Lwanga, Bernardine Mutanu and Elizabeth Merab