Diseases can spread in different ways, depending on the disease!
Some spread through close contact – for example if someone has a cold or flu, they cough and the virus gets trapped in droplets of spit (yuck), then someone else might inhale it. Another form of direct transmission is sexually transmitted diseases, where the organism passes directly from one persons skin to another.
Some spread through indirect contact, for example someone might cough into their hand, then open a door and leave some of the viruses on the door handle – then the next person could pick up the virus. Another example might be if someone contaminates some food they are making with some of their poo (this is easier than it might sound!) and they have a gastro-intestinal disease, then the person eating the food might get infected with the disease.
Others spread indirectly, so they need another thing to be able to pass from person to person. An example of this would be malaria, which needs to be picked up from one infected person by a mosquito, and then passed on to another person when a mosquito bites them. Or zoonotic diseases, where they are passed on to people from animals. Another example is schistosomiasis, which is a little worm – it has to live in a small water snail for part of its life to become infective to people – if there is no snail, then there is no transmission to people.
Disease transmission can be pretty complex, but it’s fascinating to learn about how some diseases alter the behaviour of people to encourage transmission. For example, rabies makes animals violent and they tend to bite more frequently when infected, which can pass on the virus. Some worms come out of the leg of an infected person (another yuck!), which really burns and makes the person want to put their leg in water – this means that the worm can burst, releasing all it’s babies into the water, which can then infect more people.
And also the diseases don’t even need to be in the human to affect another human. This is why we wear full protective clothing in the lab – lab coat, gloves and sometimes goggles. I work with fluids that have come from the body, and these fluids have not had any preservative added to them. We call this ‘unfixed’. When they are unfixed they can make us sick if we are not sensible and careful about our work.
When we deal with unfixed fluids, we assume that they are all high risk, and sometimes the ward staff tell us that they definately are high risk – they add a label to the form ‘Danger of Infection’, and sometimes they forget!!
When they write on the form what they think the disease might be- ?TB, ?PCP – we can tell if this means they are danger of infection, even if they have forgotten the label (which is pretty often)! We treat these specimens with heightened care.