1.   Some of the DNA, including the genes that produce the toxins, appears to have been implanted by viruses.

2.   The virus implants itself in the intestine from where it can be excreted for three to four months before it is finally eradicated by the body.

3.   The virus implants itself in the intestine where it can last for three to four months before finally eradicated by the body.

4.   The virus implants itself in the intestine from where it can be excreted for three to four months before it finally eradicated by the body.