A simple science fact should quash all the conspiracy theories about Hillary Clinton's pneumonia

Over the weekend, Democratic presidential candidate Hillary Clinton was seen suddenly leaving a memorial for the victims of the attacks of September 11, 2001 and stumbling into a van nearby, prompting questions about her health.

Clinton, who was initially thought to be having health problems related to allergies, was officially diagnosed with pneumonia on Friday, according to a statement from her doctor, Dr. Lisa R. Bardack. The Democratic candidate has since been put on antibiotics and directed to rest.

"While at [Sunday's] event, she became overheated and dehydrated. I have just examined her and she is now re-hydrated and recovering nicely," Bardack said.

Some have decried the new diagnosis as signs that the campaign has something to hide:

But there's a very simple reason that's likely not the case. Pneumonia shares symptoms with a handful of other illnesses, including the common cold, making it difficult to diagnose. As a result, it's not entirely unfathomable that Clinton's doctors may have initially treated her for another illness. According to her doctor, Clinton was previously being treated for a cough related to allergies, the BBC reported.

What is pneumonia?

Pneumonia is typically characterized by swelling of the tissue in one or both lungs. It is most often caused by a bacterial infection, but viruses or fungi that target the lungs can also trigger it. People with pneumonia often have trouble breathing because the infection causes tiny air sacs at the end of the breathing tubes in the lungs to fill with fluid.

In some cases, pneumonia symptoms can show up very quickly - possibly what happened in Clinton's case - over the course of one to two days, or they may develop more slowly over several days. Those symptoms typically include:

• a cough, which can be dry, or produce mucus (phlegm)
• trouble breathing, even when at rest
• quick heartbeat
• fever
• sweating and shivering ("the chills")
• diminished appetite
• chest pain, which is made worse by coughing

What are the complications of pneumonia?

Aside from the bacterial pneumonia, the most common form of the illness, there are four other types of pneumonia which can, in some cases, be more serious.

Fungal pneumonia, for example, which is caused by fungi, typically affects people whose immune systems have been weakened by other disorders or diseases including AIDS.

Hospital-acquired pneumonia, on the other hand, can develop in people during hospital stays. People who need breathing machines or are in intensive care, for example, are at particular risk.

But most often, a case of mild pneumonia can be treated easily at home with rest, antibiotics, and drinking plenty of fluids.

Typically, only at-risk groups - young children, older people over 65, and people with pre-existing health problems like diabetes - need to be concerned with complications arising alongside a case of pneumonia.

Clinton is 68, so she technically falls into one of these categories. If she has other health problems as well, her doctors might decide her condition is more serious, but for now, they seem to be treating it as if it were a mild case.

While pneumonia is often relatively harmless in people who are otherwise healthy, it is a killer in parts of the developing world. Globally, pneumonia is responsible for 15% of deaths in children under five each year. The World Health Organization calls it the single biggest killer of children in this age group.