It seems like every few years, a new disease comes along and spreads like wildfire, leaving a trail of death and devastation in its wake. Are we getting worse at fighting off infections, or are they getting better at invading our bodies?
A lot of the diseases that humans tend to catch regularly, like the common cold or flu, have actually evolved with us over millions of years. This is called co-evolution. Just as we evolve to battle them a little better, they evolve to attack in a slightly different way that bypasses our new defence – it’s an ongoing arms race.
Lots of bacteria and viruses have developed very clever ways of avoiding our immune systems so they can survive in our bodies. Some are able to go into hiding, like the herpes virus (spread by physical contact) which causes cold sores. This hides in the large junctions between nerve cells (called ganglia) in the head and spine, waiting for you to get ill or stressed. It then travels back towards your face and causes those unmistakable blisters.
Others are attacked and taken up by white blood cells because of the immune response, but are able to escape. Instead of being destroyed by the white blood cell like normal, they go on to cause further illness. This strategy is used by a particularly nasty bacterium called Shigella dysenteriae (spread by contaminated water and poor hand washing practices) – this infects the large intestine and causes severe bloody diarrhoea.
Different animals have different types of cell surface signatures that microbes either can or can’t interact with. If it can, then the germ can attach to or get inside of the cell and cause illness – like having the right key to unlock a door. Bird cells only have a single type of signature, but pig cells have their own different type of signature as well as the same sort of signature as birds.
This means pigs can be infected by both pig and bird diseases, and these diseases can then interact with each other. Additionally, the signature found only on pig cells is actually compatible with humans. So pigs can act as mixing bowls of bird (avian) and pig (swine) illnesses, creating new diseases and then passing them on to humans. This is actually one way that new kinds of flu are ‘made’ or evolve. When these new strains crop up, our immune systems don’t quite know how to deal with them at first, and depending on how unpleasant the virus is, they may even cause death.
Anything that tests a disease’s ability to survive in your body can provoke evolutionary changes. This is how antibiotic resistance starts. Inappropriate use of antibiotics – that is doctors having prescribed them when they aren’t needed, and/or patients not taking them properly – can expose germs to amounts of antibiotic that aren’t strong enough to kill them.
As the saying goes, what doesn’t kill you makes you stronger! The bacteria are able to get used to the antibiotic and develop immunity or resistance. This is how ‘superbugs’ like MRSA (often found in hospitals) and multidrug-resistant tuberculosis (MDR-TB) come about. MDR-TB is a particular problem because tuberculosis is usually treated with combinations of very strong antibiotics over a long period of time, these drugs are kept in reserve – used only for this especially difficult-to-treat disease. That means it’s quite scary that TB is evolving resistance, this is why drug companies are searching for new and improved antibiotics.
Although we humans may think of ourselves as being very advanced, our species went through a ‘population bottleneck’ tens of thousands of years ago. This means a significant proportion of the human population at the time was wiped out, and their genes were lost. The resulting loss of genetic diversity means that modern day humans are incredibly similar in terms of their genes, and we have to wait for random mutations to occur in order for evolution to take place. So our ability to evolve to prevent or better combat infectious disease, is much slower when compared to the speed bacteria, viruses and infectious fungi can evolve at.
One notable human mutation in this context is sickle cell disease. Although the disease itself (which is genetic, not infectious) has lots of unfortunate symptoms, in some cases it does in fact give protection from malaria. This is because the parasite which causes malaria makes sickled cells collapse after infection.
As progressive as our technologies and innovation may be, there could soon come a time when we run out of antibiotics, or we cannot adapt fast enough to fight these newly emerging diseases. Could it be possible that we might lose this arms race?
Article by Catherine Butterfield of University Birmingham for Blog About Healthcare