What did the Prophet (peace and blessings of Allaah be upon him) mean by “No contagion (‘adwa)”?
Question

What is the original text of the hadeeth “There is no ‘adwa in religion” and what is meant by that?.
Praise be to Allah.There are several versions of this hadeeth. Al-Bukhaari (5776) and Muslim (2224) narrated from Anas ibn Maalik (may Allaah be pleased with him) that the Prophet (peace and blessings of Allaah be upon him) said: “There is no ‘adwa (transmission of infectious disease without the permission of Allaah) and no tiyarah (superstitious belief in bird omens), but I like good omens.” They said: What is a good omen?” He said: “A good word.” 
Al-Bukhaari (5316) narrated from Abu Hurayrah (may Allaah be pleased with him) that the Prophet (peace and blessings of Allaah be upon him) said: “There is no ‘adwaa [contagion, transmission of infectious disease without the permission of Allaah], no tiyarah [superstitious belief in bird omens], no haamah [refers to a Jaahili Arab tradition described variously as: a worm which infests the grave of a murder victim until he is avenged; an owl; or the bones of a dead person turned into a bird that could fly], and no Safar [the month of Safar was regarded as “unlucky” during the Jaahiliyyah].” 
Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) said:  
The words of the Prophet (peace and blessings of Allaah be upon him) “There is no ‘adwa” are general in meaning, thus the Prophet (peace and blessings of Allaah be upon him) states that there is no contagion (without the permission of Allaah). 
‘Adwa (contagion) refers to the spread of a disease from a sick person to a healthy one. What happens in the case of physical diseases may also happen in the case of psychological diseases. Hence the Prophet (peace and blessings of Allaah be upon him) said that a bad companion is the like one who operates the bellows: Either he will burn your clothes, or you will notice a bad smell from him. 
The words “there is no ‘adwa” include both physical and psychological diseases, even though it is more apparent with regard to physical disease. 
The words “no tiyarah (lit. superstitious belief in bird omens)” refers to feeling superstitious because of something that you see, hear or know.. 
With regard to the words “no haamah”, haamah is interpreted in two ways: 
1 – That it is a bird similar to an owl, or an owl. The Arabs believed that if a person was killed, his bones became a haamah that could fly and shriek until he was avenged. Some of them believed that the haamah was the soul of the slain person. 
2 – Some of the Arabs said that the haamah was a particular bird, which they regarded as a bad omen. If it landed on the house of one of them and made a sound, they said that it was predicting a death. They believed that this was a sign that that person would die soon. All of these are undoubtedly false beliefs. 
The words “No Safar” refers to the month of Safar, which the Arabs used to regard as inauspicious, especially for marriage. 
And it was said that it was a stomach disease that affected camels and was transmitted from one camel to another; based on this, mentioning it after ‘adwa (contagion) comes under the heading of mentioning something specific after something general. 
It is more likely that what is meant by Safar here is the month, and that what is meant by saying “No Safar” is that it is not to be regarded as inauspicious, rather it is like any other time, during which good or bad things may be decreed. 
This does not mean that these things do not happen, because they do happen. Rather it is a statement that they do not have any effect. The One Who causes things to happen is Allaah. If any of these things has a known effect then that is valid and real; if any of them is imagined to have an effect, then that is invalid and false. Thus the hadeeth demonstrates that if there is a valid effect, it cannot be attributed to that thing itself (rather the One Who causes it to happen is Allaah). If the effect is merely imagined, then this hadeeth is stating that it has no effect in the first place. 
With regard to the words “no contagion (‘adwa)”: contagion is something that happens, as is indicated by the words of the Prophet (peace and blessings of Allaah be upon him), “Do not put a sick one with a healthy one” i.e., the owner of a sick camel should not bring it to the owner of a healthy camel, lest the contagion be transmitted. 
And the Prophet (peace and blessings of Allaah be upon him) said: “Flee from the leper as you would flee from a lion.” Leprosy is a serious disease that is transmitted quickly and kills the one who catches it. It was even said that it is a plague. Hence the command to flee from the leper so that the disease will not be transmitted from him to you. This is an affirmation of the effect of contagion, but its effect is not inevitable in such a way that it affects people by itself. The command of the Prophet (peace and blessings of Allaah be upon him) to flee and not to bring sick camels to where healthy camels are comes under the heading of avoiding the means (that lead to sickness), not attributing the effect to the means themselves. The means do not affect anything themselves, but we should avoid the things that may be a cause of calamity, because Allaah says (interpretation of the meaning): 
“and do not throw yourselves into destruction”
[al-Baqarah 2:195]
We cannot say that the Messenger (peace and blessings of Allaah be upon him) was denying the effect of contagion, because contagion is something that is proven to exist in real life and is mentioned in other ahaadeeth. 
When the Prophet (peace and blessings of Allaah be upon him) said “there is no ‘adwa”, a man said: “O Messenger of Allaah, the camels may be healthy like deer, then a mangy camel comes and mixes with them and they all get the mange.” The Prophet (peace and blessings of Allaah be upon him) said: “And who infected the first one?” meaning that the disease came to the first one with no contagion, rather it came from Allaah. By the same token, if it was transmitted by contagion, then it was transmitted by the command of Allaah. A thing may have a known cause or it may not have a known cause. The mange of the first one does not have a known cause, apart from the fact that it happened by the will and decree of Allaah, and the mange that came after it does have a known cause. But if Allaah wills the camel will not become mangy. Hence sometimes a camel may get the mange but then it recovers and does not die. The same happens with the plague and cholera; they may enter a house and some may get sick and die, while others are not affected at all.
 We have to put our trust in Allaah and depend on Him. It was narrated that a leper came to the Prophet (peace and blessings of Allaah be upon him) and he took him by the hand and said, “Eat,” i.e., eat of the food that the Messenger (peace and blessings of Allaah be upon him) was eating– because of the strength of his trust in Allaah. This trust counteracts the causes of contagion. 
What we have referred to above is the best way of reconciling between the ahaadeeth. 
End quote from Sharh Kitaab al-Tawheed, 2/80 
Based on this, what is meant by the words of the Prophet (peace and blessings of Allaah be upon him) “no ‘adwa (contagion)” is that disease is not transmitted from a sick person to a healthy one by itself, rather it is transmitted by the will and decree of Allaah. If a sick person mixes with healthy ones this is one of the causes of the transmission of disease. But this does not mean that it will inevitably happen, rather it only happens if Allaah wills it. Hence we often see a sick mother give birth to a healthy child, but the disease is not transmitted to the infant. 
And Allaah knows best.