For most parents it comes as a shock to discover that their child has got head lice.
It usually comes as an even bigger shock to find that their child hasn't just go one or two lice but is seemingly crawling with them.
If this has happened to you then you can stop feeling like a terrible, neglectful parent who should have spotted the lice earlier.
You've come up against one of the most successful parasites when it comes to man.
Many other of man's parasitical foes are now rarely encountered in modern society as better personal hygiene has spread over the past few decades, but the head louse still continues to thrive.
Unlike other parasites that have infested humans the fact that we now keep clean has no impact on Pediculus Humanus Capitis as head lice are scientifically known.
Clean hair or dirty hair they really don't mind.
And washing the hair neither dislodges them nor drowns them.
Of course, as a parent you're bound to think that you should have noticed the lice earlier on but head lice aren't designed to be spotted easily.
They wouldn't have thrived for thousands of years if they could be seen with just a casual look.
Their colouration, their size and the way they move through the hair are all designed to make them hard to spot.
Unless you are actually looking for them you are unlikely to see them until the hair has become heavily infested.
As for the eggs, or nits as they are called, they are even more difficult to see and it can be very difficult to tell the difference between a nit and a flake of scalp skin if you've never seen a nit before.
So what about all the scratching that goes with head lice, surely you should have noticed that.
Although this is often an indication of the presence of lice only about a third of people infected will scratch.
The itching is caused by an allergic reaction to the bite, or more specifically the saliva produced during the bite.
It's thought that it takes several hundreds, if not thousands, of bites before the scalp of someone who is allergic to louse saliva will become itchy enough for them then to begin scratching.
By this time the hair will already have a thriving colony of head lice living in it.
Because of all this it is good idea to have a routine of regular checks for head lice during the vulnerable school years when the chances of catching them are at their greatest.
You only need to check the hair once or twice a month unless there are rumours of an outbreak when you'll want to do it at least once a week.
Get yourself a good head lice comb and work methodically through the hair inspecting the comb as you go.
With a bit of practice you should be able to do this quite quickly.
By carrying out regular checks you can prevent a lot of tears and embarrassment all round.