Millions of people affected by flooding in South Asia could face a health crisis, experts have warned.
The flooding in South Asia could have serious health implications
The World Health Organization (WHO) and Unicef have said stagnant waters can provide "a lethal breeding ground" for diseases such as malaria and dengue fever.
Sandy Cairncross, professor of environmental health at the London School of Hygiene and Tropical Diseases, said flooding could cause an increase in certain diseases, particularly diarrhoeal diseases.
These diseases include cholera and typhoid, and can cause dehydration which, if not treated, can be very serious.
Public health risk
Professor Cairncross said: "Lots of bugs can cause these diarrhoeal diseases, and we can't tell which has done so without using microbiology tests in the lab.
"But many of these bugs can be waterborne, and they can be spread through person-to-person contact, so if people are not able to maintain levels of hygiene and are living in crowded shelters after flooding, disease rates can increase."
He said the biggest threat to public health was likely to come from agents such as rotaviruses, which may cause less dramatic symptoms than cholera or typhoid, but which were likely to affect many people.
He added that such diseases could be fatal in children and so posed a "serious public heath risk."
And diseases spread by mosquitoes, such as malaria, could also be a risk.
Professor Cairncross said: "Mosquitoes need still water to breed, so as flood waters recede and puddles form, this can provide a breeding ground for them."
Cholera is an intestinal infection caused by bacteria, and it is often linked to contaminated drinking water supplies.
The bacteria can spread quickly through water supplies via the faeces of victims, especially where there is poor sanitation.
It causes severe diarrhoea and vomiting, and until clean water supplies and systems to dispose of human waste safely are brought in, outbreaks cannot be controlled.
Hand washing after going to the toilet and cooking food correctly are vital to prevent its spread.
Typhoid fever is caused by a bacteria which can be spread by eating or drinking products contaminated by another victim, or if contaminated sewage gets into water used for drinking or washing food.
Polluted water is the most common source, so the disease is a problem in flooded areas if water supply and sewage removal are disrupted.
Symptoms include sudden onset of fever, severe headache, nausea, and diarrhoea, and cause a one in ten risk of death without treatment.
Antibiotics may be used to treat patients, although resistance to these drugs can be a problem.
Gastroenteritis is caused by a type of virus, the rotavirus which can be spread very easily, particularly between children, who are unlikely to have developed immunity to it yet.
The virus can be passed on by coming into contact with infected patients, or even their clothes, and by breathing in infected airborne particles.
Symptoms include vomiting, diarrhoea, and stomach cramps, and in extreme cases fever and convulsions.
It can also lead to dehydration if untreated, which can be dangerous particulary if there is not enough drinking water available.
Malaria kills over a million people a year, and is a parasitic disease spread via certain mosquitoes.
These mosquitoes need stagnant, shady water in order for their eggs to develop - conditions which are abundant during times of flooding, particularly as the waters recede leaving puddles.
Most people survive a bout of malaria after a 10-20 day illness, but symptoms, which include cycles of a high fever followed by chills, must be detected early.
The most serious forms of the disease can affect the kidneys and brain, causing complications such cause anaemia, coma and even death.
Dengue fever is caused by a virus spread by certain strains of mosquito.
It causes sufferers to ache all over, and they may feel like their bones have all been broken.
If people are infected with more than one strain of the virus their immune system can overreact leading to internal bleeding and even death.
The mosquitoes responsible for spreading the disease lay their eggs near water, and when water levels rise this washes the eggs away so they can then develop.
If flood waters are rising and falling, this could provide new breeding sites for the mosquitoes.