Scientists are closer to understanding why a common parasite is harmless to most people, while causing severe illness in others.
Pregnant women are at particular risk from toxoplasmosis
Toxoplasma is carried by cats and rats in the UK, and a large proportion of humans are also thought to carry the parasite, without any ill effects.
But it can cause toxoplasmosis, which can lead to brain damage or even death.
Stanford University scientists, writing in Nature, say the behaviour of a single protein determines what happens.
Toxoplasma is particularly dangerous for people with weakened immune systems, such as those with HIV, and for women in the first three months of pregnancy, as it can cause severe birth defects.
Its normal lifecycle starts in cats. It is then passed, usually via the cat's faeces, to rats and then back to cats when they catch and eat the infected rats.
Humans can become infected either when they come into contact with cat faeces, or by eating undercooked mutton, as sheep can also become carriers.
The Stanford researchers are interested in the ability of Toxoplasma to adapt genetically to a wide variety of hosts.
Their study reveals how the parasite injects a single protein into a cell it wants to invade, and how this protein makes its way to the nucleus of the cell and interferes with the ability of the cell to trigger an immune defence.
Susan Coller, one of the researchers leading the project, said that the strategy was very effective: "The nucleus is the heart of the cell, the ultimate prize. If you want to affect the cell in a dramatic way, go straight there."
The other finding was subtle differences in this key protein between different types of Toxoplasma - each different strain perhaps tailored to infecting different types of host cell with the minimum damage.
Severe toxoplasmosis may happen when the "wrong" strain, one not suited to infecting humans, tries to invade our cells with the protein either too powerful - overwhelming and destroying them, or ineffective - triggering a massive immune system response.
They suggest that their discoveries about how Toxoplasma is so successful could also apply to our parasite invaders - such as malaria.
Professor John Barrett, a senior lecturer in parasitology at the University of Aberystwyth, said that there were arguments for screening pregnant women for Toxoplasma to avoid birth defects.
"The effect on the foetus can be very serious and there is a reliable test for Toxoplasma, so in these higher risk groups, it may be worthwhile."