Recently, I have been fielding a whole slew of questions about the Swine Flu. To really explain this, I have to start from the beginning.
1) The flu = an illness caused specifically by an influenza virus. This is a type of virus like a whale is a type of sea mammal. If you catch the HIV virus, you have HIV, not the flu. Get it?
2) The genetic code of the influenza virus is RNA. It does the same job for the virus as our DNA does for us: it is the code of the genes (for the purposes of our discussion).
The news media has been explaining that the swine flu came about by evolving. This is very true, but it is confusing since we are taught that evolution takes millions upon millions of years and yet, here swine flu suddenly is. Let's start with human reproduction. One copy of every gene from the mother is carried in the egg, one copy from the father in the sperm. This allows for recombination of our genes with each offspring. Every child has half of its genes from each parent.
Viruses do not do this -- they self-replicate by infecting a cell, making millions of copies of itself and then breaking free of the cell to go infect more. This means one "parent" makes millions of "offspring" which are identical to the parent. Not only that, but when the replicate themselves, they hijack the cell's machinery for building RNA and proteins. Like an assembly line, all the different parts of the virus get made separately and then at the very end, everything gets packaged together into a new virus. (see below. Yellow = virus, red squiggles = RNA... remember that for later.)
How do humans evolve?
2) Mixing genes which is called "Genetic recombination" (remember, 1/2 of each parents genes --> one offspring)
How can viruses evolve?
1) Mutation! however, mutation is a slow, slow process. In humans, it can take hundreds of generations for a mutation to spread throughout the population. It takes only a few generations in a virus (remember-- ONE parent --> TONS of offspring, no genetic mixing) AND those generations happen on the order of hours to days, not centuries or millenia. Still, too slow for the sudden emergence of the swine flu.
2) Viruses have figured out a way to have genetic recombination despite having a single parent per offspring. This is where subtypes come into play. Let me explain...
Earlier I said that the influenza virus is a type of virus like a whale is a type of sea mammal. But if you follow that out again, there are lots of types of whales -- humback, blue, killer... etc. Similarly, there are lots of subtypes of influenza viruses and these are classified by H and N. The swine flu is in the H1N1 subtype. The bird flu is H5N1. Different kinds of whales, but still whales. (Interesting side note, it was an H1N1 virus, the Spanish Flu, that caused the pandemic of 1918.)
Let's go back to looking at our simple virus diagram on the left. Remember that the red squiggles are RNA? They look like they are free-floating in the cell. They basically ARE! Now, imagine that the same cell got infected with two different influenza viruses at the same time. Those viruses would just have their RNA floating around free in the cell! When the virus puts itself back togehter, it grabs whatever RNA is sees... leading to a mixutre and a NEW type of influenza virus.
This is how swine flu came about so suddenly. A host, be it animal or human, was infected with multiple kinds of influenza virus at the same time. The viruses got scrambled inside the cell and out came a new kind of virus or, more accurately, LOTS of new kinds of viruses because each individual virus is going to have a random assortment of RNA from the two parent viruses (much like humans). Most of the new viruses formed from this method (called reassortment and recombination) will not live. However, with so many combinations, nature is bound to hit a jackpot at least once. Voila swine flu.
In fact, the swine flu virus has genes from so many different types of influenza virus that science is baffled as to how it got so many! Researchers are counting RNA from 3 different swine viruses, bird and human in the swine flu. As a result, this virus is something very novel that our bodies are not used to defending against. It doesn't look like other influenza viruses to our immune system, so it takes longer for our bodies to fight it off. This is accounting in part for why swine flu is so contagious.
The bad news: the flu vaccine will not protect you.
The good news: antiviral drugs (Tamiflu and Relenza) will work.
The bad news: antiviral drugs are not very effective after symptoms start, which is why they are not commonly used in medical practice.
I mainly used my memory to write this one, but here is an early and easy to understand editorial about swine flu.
Butler D. Swine Flu Goes Global. Nature 2009 Apr 30;458(7242):1082-3.
Next time: Do you have Swine Flu?