I dont have the correct english word for this. So I am going with the same tamil word Theetu (if you know the word that exactly describes it, please update me). Theetu in broader terms means being unholy (due to child birth, or your menstrual cycle, or a death in a family etc etc)
Disclaimer:
I may be slightly incorrect with the number of days of observing theetu in these occasions as I am not fully aware of the exact rituals that are followed, because I am not interested in that aspect. I am more interested in the 'why' part of the ritual.
Ok, Let us start from Child birth:
After the child birth, Theetu is applicable for the mommy's ( daddy's are spared here) and they shouldnt go to temple, or light a lamp etc. And this is applicable for almost 10 days and when the Punyajanam is done on 11th day, then only the mommy's are cleared of their theetu. (I hope I am right about the days here, having recently attended these functions :) )
-> Ok, this might sound a little stupid if you dont believe in it, but there is a certain scientific reason that this was followed for so many years. Typically during child birth, or after it, both the mom and the kid will be very weak for a few weeks. The happy mommy will have lost so much of strength (especially in case of normal deliveries) and will be more suspectible for any kind of viral attacks which might spoil their health. Now if only the mommy's are healthy, peaceful and calm, they can get their strength to feed and nourish their kiddos. And the initial few weeks of good feeding of mother's milk is very essential for the immune system of the kid. If there wasnt any concept of theetu, what would have happened was, within a day or two after the child birth, the mommy's will be forced to work again and take care of the whole household and hence it might have had a bigger impact on herself and the kid's health. So our good old elders devised a plan or rather a ritual sort of thing to ensure that the mom and child are healthier, by isolating them. Generally 16-20 days is required for the mom to be back to her normal self (with respect to health). I guess now you would have known the reason why daddy's arent associated with the theetu :) [Boy, they really dont loose anything do they ?].
Next comes the menstrual cycle for the girls: Generally during the menses time girls follow theetu for 3 days. Again here the reason is that girls need more rest as the body undergoes heavy loss of stamina (especially in form of iron and heamoglobin). They may be in need for more oxygen too. There is also the body odour due to the changes in the hormones of the girls. In order to give them complete rest, this theetu was formed. Earlier clothes were used for sanitary napkins which was more unhygenic and hence it was always better to be isolated and not spread any kind of viruses to others. That is the funda as far as I know. In this age, does this apply still ? Well again that depends on where you are , how much of hygiene ways you follow etc etc.. If you take care of yourself you may not need this theetu do you ? What will people who live alone do during such theetu..obviously not starve to death right ?
Last but not least , the death: In cases of death, you follow theetu for almost 10 days and if you happen to visit the house where death has taken place or you see the dead body you follow theetu when you come back home and not touch anything until you bathe it off. The reason being that the presence of the dead body will expose us to various bacterias (which would be working upon the already decomposing body) and if you visit that house and come back , a good bath is the one that can get you rid of those,. That is why they ask you to bathe first before you start spreading it to everything / everyone else. Its the same case if there is a death in the house, they say it takes around 10 days for the cleansing because you have been there through out till cremation.
Ok, now having said the basic reasons for these kind of theetu's (I know there are many more..but these form the basics) and as to why they were done (as far as I have heard or known) it is now left to your common sense to apply or disregard the same. Let us not reject everything as baseless or unnecessary and not accept everything without knowing as to why they were said/done in the first place.
Do not believe in anything simply because you have heard it.
Do not believe in anything simply because it is spoken and rumored by many.
Do not believe in anything simply because it is found written in your religious books.
Do not believe in anything merely on the authority of your teachers and elders.
Do not believe in traditions because they have been handed down for many generations.
But after observation and analysis, when you find that anything agrees with reason and is conducive to the good and benefit of one and all, then accept it and live up to it.
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Buddha (563 - 483 BC)